HACKETT SPLEEN CLASSIFICATION A numerical means of recording the size of an enlarged spleen, especially in malaria. This is a six-point scale of 0 (no enlargement) to 5 (enlarged to umbilicus or larger).

HALF-LIFE Time in which the concentration of a substance (especially if radioactive) is reduced by 50%.


  1. The influence upon an observation of the observer’s perception of the characteristics of the individual observed (other than the characteristic under study). The influence of the observer’s recollection or knowledge of findings on a previous occasion.
  2. The effect (usually beneficial) that the manner, attention, and caring of a provider have on a patient during a medical encounter regardless of what medical procedures or services the encounter involves. See also placebo effect.

HANDICAP Reduction in a person’s capacity to fulfill a social role as a consequence of an impairment or disability, inadequate training for the role, or other circumstances. Applied to children, the term usually refers to the presence of an impairment or other circumstance that is likely to interfere with normal growth and development or with the capacity to learn. See also international classification of impairments, disabilities, and handicaps for the official WHO definition.

HANDICAP-FREE LIFE EXPECTANCY The average number of years an individual is expected to live free of handicap if current patterns of mortality and handicap continue to apply.123 See also disability-free life expectancy; health expectancy.

HAPHAZARD SAMPLE Selection of a group for study without thought as to whether they are representative of the population. The word haphazard here implies selection based on a mixture of criteria such as convenience, accessibility, turning up at the time an investigation or study is in progress, belonging to some existing list or registry, etc. Because they have an unknown chance of being unrepresentative of the population, haphazard samples are unsatisfactory for generalization.

HARDY-WEINBERG EQUILIBRIUM State in which the allele and genotype frequencies do not change from one generation to the next in a population.23,210 Although condi- tions for Hardy-Weinberg equilibrium are seldom strictly met, genotype frequencies are often consistent with the Hardy-Weinberg law. Several software packages exist to test whether a set of genotypic frequencies are in Hardy-Weinberg equilibrium.

HARDY-WEINBERG LAW The principle that both gene and genotype frequencies will remain in equilibrium in an infinitely large population in the absence of mutation,


Harmonic mean 110

migration, selection, and nonrandom mating. If p is the frequency of one allele and q is the frequency of another and p + q = 1, then p2 is the frequency of homozygotes for the allele, q2 is the frequency of homozygotes for the other allele, and 2 pq is the frequency of heterozygotes.

HARMONIC MEAN See mean, harmonic.
HAWTHORNE EFFECT The effect (usually positive or beneficial) of being under study

upon the persons being studied; their knowledge of the study often influences their behavior. The name derives from work studies by Whitehead, Dickson, Roethlisberger, and others, in the Western Electric Plant, Hawthorne, Illinois, reported by Elton Mayo in The Social Problems of an Industrial Civilization (London: Routledge, 1949).


  1. Inherent capability of an agent or a situation to have an adverse effect. A factor or exposure that may adversely affect health. Loosely, in lay speech a synonym for risk; in epidemiology, a similar concept to risk factor.
  2. (Syn: force of morbidity, instantaneous incidence rate) A theoretical measure of the probability of occurrence of an event per unit time at risk; e.g., death or new disease, at a point in time, t, defined mathematically as the limit, as t approaches zero, of the probability that an individual well at time t will experience the event by t + t, divided by t.


  1. The World Health Organization (WHO) described it in 1948, in the preamble to its constitution, as: “A state of complete physical, mental, and social well-being and not merely the absence of disease or infirmity.”
  2. In 1984, a WHO health promotion initiative led to expansion of the original WHO description, which can be abbreviated to: “The extent to which an individual or a group is able to realize aspirations and satisfy needs, and to change or cope with the environment. Health is a resource for everyday life, not the objective of living; it is a positive concept, emphasizing social and personal resources as well as physical capabilities.”223
  3. A state characterized by anatomical, physiological, and psychological integrity; ability to perform personally valued family, work, and community roles; ability to deal with physical, biological, psychological, and social stress; a feeling of well-being; and freedom from the risk of disease and untimely death.224
  4. A state of equilibrium between humans and the physical, biological, and social environment compatible with full functional activity.225 A sustainable state in which humans and other living creatures with which they interact can coexist indefinitely in equilibrium. Health is derived from the Old English hal, meaning whole, sound in wind and limb. See also sustainability.

HEALTH-ADJUSTED LIFE EXPECTANCY Life expectancy expressed in quality- adjusted life years. See health expectancy.

HEALTH BEHAVIOR The combination of knowledge, practices, and attitudes that together contribute to motivate the actions we take regarding health. Health behavior may promote and preserve good health or, if the behavior is harmful (e.g., tobacco smoking), may be a determinant of disease. This combination of knowledge, practices, and attitudes has been described and discussed by several writers.226 See also disease; illness behavior; sickness “career.”

111 Health gap

HEALTH CARE Services provided to individuals or communities by agents of the health services or professions to promote, maintain, monitor, or restore health. Health care is not limited to medical care, which implies action by or under the supervision of a physi- cian. The term is sometimes extended to include health-related self-care.

HEALTHCARE EPIDEMIOLOGY See hospital epidemiology.
HEALTH DEVELOPMENT A collective effort to improve health and well-being in all

individuals and communities of a society, taking into account prevailing political, cul-

tural, social, and economic features.
HEALTH EDUCATION Learning resources and teaching programs concerned with

health—its protection and promotion. It often begins in the family and continues through primary and secondary education, with emphasis on exercise, diet, care of teeth, avoidance of sexually transmitted disease, sexuality, social relationships, smoking, alcohol, and other drugs, accidents, violence, etc. Health education may be provided by school teachers and nurses as well as by specially trained educators or physicians. It is also conducted in the community and with subsets of the population, including pregnant women, workers, people about to retire, or the elderly. See also acquaintance network; context; prevention.

HEALTH EQUITY Fairnes and impartiality in any health-related determinant (e.g., expo- sure, policy) or outcome. Equity in epidemiologic risk management aims to ensure that communities near sites hazardous to health (e.g., polluting industrial facilities) are not more exposed to such environmental health risks than are more affluent communities removed from the source of pollution. Achieving equity of access to health care services regardless of social, ethnic, and cultural status has become a high-priority health policy issue in many countries.85–87,137,138,221

HEALTH EXPECTANCY A general term for several health indicators in which life expectancy is weighted for health status. The term refers to the average amount of time (years, months, weeks, days) a person is expected to live in a given health state if cur- rent patterns of mortality and health states continue to apply; the patterns are derived from epidemiological and vital statistics data. Health expectancy is therefore a statisti- cal abstraction, based on existing age-specific death rates and age-specific prevalences for health states or on age-specific transition rates between health states.227 Specific health expectancies are based on health states defined by the International Classifi- cation of Impairments, Disabilities and Handicaps (ICIDH) concepts of impairment, disability, and handicap. Examples include disability-free life expectancy and handicap- free life expectancy. Health expectancy – also called life expectancy in good health or healthy life expectancy and disability-free life expectancy – considers only the time spent in good health. Health-adjusted life expectancy and disability-adjusted life expectancy assign values to ranges of health states.

HEALTH FOR ALL The cultural, social, and political objective of health policy, enshrined in the WHO Alma-Ata Declaration (1978). It was interpreted as a goal to be achieved by the year 2000, as a slogan, and as an aspiration that might be realized by implement- ing primary health care for all citizens of a country.

HEALTH GAP General term for a group of health indicators in which lost life expectancy is weighted by health status. They may be distinguished on the basis of a specified or implied health target or norm for a population, definition and weighting of health states, and inclusion or exclusion of values other than health. Unlike health expectancies,

they can be computed for specific causes of mortality and morbidity. potential years of life lost and expected years of life lost use a target selected arbitrarily or drawn from a life table and give all remaining years of life equal weight; disability-adjusted life years (DALYs) assign weights to years of life remaining.

HEALTH IMPACT ASSESSMENT (HIA) See public health impact assessment. HEALTH IN ALL POLICIES (HIAP) A theme or slogan for intersectoral action, hori- zontal health policies and healthy public policies. The main health theme of the Finn- ish European Union Presidency in 2006 (www.stm.fi). A strategy to help strengthen links between health and other policies. HiAP addresses the effects on health across all policies (such as agriculture, education, the environment, fiscal policies, housing, and transport). It seeks to improve health and at the same time contribute to the well-being and the wealth of the nations through structures, mechanisms, and actions planned and managed mainly by sectors other than health. An approach that promotes coordination mechanisms to ensure that the health dimension is integrated into activities of all gov- ernment agencies and services. While the health sector has gradually increased its coop- eration with other government sectors, industry, and nongovernmental organizations in the past four decades, other sectors have increasingly taken health and the well-being of citizens into account in their policies. The key factor enabling such a development has been that health and well-being are shared values across societal sectors. HiAP is a political result of the growing recognition of the importance of health for the overall objectives of a society: health is a key foundation stone of strategies of growth, com- petitiveness, and sustainable development. The HiAP approach uses an integrated

approach to health impact assessment (HIA).
HEALTH INDEX A numerical indication of the health of a given population derived from

a specified composite formula. The components of the formula may be infant mortality

rates, incidence rate for particular disease, or other health indicator.
HEALTH INDICATOR A variable, susceptible to direct measurement, that reflects the state of health of persons in a community. Examples include infant mortality rates,

113 Health status index

incidence rates based on notified cases of disease, disability days, etc. These measure-

ments may be used as components in the calculation of a health index.
HEALTH PROMOTION The process of enabling people to increase control over their health and improve it. It involves the population as a whole in the context of their everyday lives rather than focusing on people at risk for specific diseases and is directed toward action on

the determinants or causes of health.228 See also prevention; preventive medicine. HEALTH RISK APPRAISAL (HRA) [(Syn: health hazard appraisal (HHA)] A generic term applied to methods for describing an individual’s chances of becoming ill or dying from selected causes. The many versions available share several common features: Starting from the average risk of death for the individual’s age and sex, a consideration of various lifestyle and physical factors indicates whether the individual is at greater or less than average risk of death from the commonest causes of death for his or her age and sex. All methods also indicate what reduction in risk could be achieved by altering any of the causal factors (such as cigarette smoking) that the individual could modify. The premise underlying such methods is that information on the extent to which an individual’s characteristics, habits, and health practices are influencing his or her future

risk of dying will assist health care workers in counseling their patients.
HEALTH SECTOR The sector of society that is concerned with and deals with all issues and services related to health, sickness, and the provision of health care to the popula-

tion. See also determinant.
HEALTH SERVICES Services performed by health professionals or by others under their

direction for the purpose of promoting, maintaining, or restoring health. In addition to personal health care, health services include measures for health protection, health promotion, and disease prevention.

HEALTH SERVICES RESEARCH The integration of knowledge from clinical, epide- miological, sociological, economic, management, and other sciences in the study of the organization, functioning, and performance of health services. Health services research is usually concerned with relationships between needs, demand, supply, use, and out- comes of health services. The aim of health services research is evaluation; several com- ponents of evaluative health services research are distinguished, namely:

  1. Evaluation of structure, concerned with resources, facilities, and manpower.
  2. Evaluation of process, concerned with matters such as where, by whom, and how
    health care is provided.
  3. Evaluation of output, concerned with the amount and nature of health services
  4. Evaluation of outcome, concerned with the results—i.e., whether persons using
    health services experience measurable benefits, such as improved survival or reduced

HEALTH STATISTICS Aggregated data describing and enumerating attributes, events,

behaviors, services, resources, outcomes, or costs related to health, disease, and health services. The data may be derived from survey instruments, medical records, and admin- istrative documents. vital statistics are a subset of health statistics.

HEALTH STATUS The degree to which a person is able to function physically, emotion- ally, and socially with or without aid from the health care system. Compare quality of life.

HEALTH STATUS INDEX A set of measurements designed to detect short-term fluctuations in the health of members of a population; these measurements include

Health survey 114

physical function, emotional well-being, activities of daily living, feelings, etc. Most indexes require the use of carefully composed questions designed with reference to matters of fact rather than shades of opinion. The results are usually expressed by a numerical score that gives a profile of the well-being of the individual.

HEALTH SURVEY A survey designed to provide information on the health status of a population. It may be descriptive, exploratory, or explanatory. See also morbidity sur- vey; cross-sectional study.

HEALTH SYSTEMS RESEARCH (Syn: health research) The multidisciplinary study of health systems, including health services research, supported by data on determi- nants of health and accurate health statistics. A term popularized by the WHO.

HEALTH TECHNOLOGY ASSESSMENT (HTA) The formal evaluation of technologies used in health care, including medicine, and in public health. It explicitly involves not only efficacy but also cost-effectiveness, cost-utility, and all other aspects of tech- nology that may be important for society. HTA supports evidence-based decision mak- ing in health care policy and practice.

HEALTHY PUBLIC POLICIES Policies that improve the conditions under which people live: such as secure, safe, adequate, and sustainable livelihoods, lifestyles, and environ- ments, including housing, education, nutrition, access to information, child care, trans- portation, and necessary community and personal social and health services.229 Policy adequacy may be measured by its impact on population health.70,196 See also health in all policies; prevention.

HEALTHY WORKER EFFECT A phenomenon observed initially in studies of occupa- tional diseases: workers often exhibit lower overall death rates than the general popu- lation, because persons who are severely ill and chronically disabled are ordinarily excluded from employment or leave employment early.230 Death rates in the general population may be inappropriate for comparison if this effect is not taken into account. Similar effects are known for military personnel, migrants, and other groups.

HEALTHY YEARS EQUIVALENTS (HYEs) A measure of health-related quality of life that incorporates two sets of preferences; one set reflects individuals’ preferences for life years or duration of life, and the other reflects preferences for states of health.231

HEALY (healthy life years) A composite indicator that incorporates mortality and mor- bidity in a single number.232 See also burden of disease; disability-adjusted life years (DALYs); disability-free life expectancy; life expectancy free from disability (LEFD); quality-adjusted life years (QALY).

HEBDOMADAL MORTALITY RATE The mortality rate in the first week of life; the denominator is the number of live births in a year.

HENLE–KOCH POSTULATES A set of causal criteria for making judgments about microorganisms as causes of infectious diseases. They were first formulated by F. G. Jacob Henle and adapted by Robert Koch in 1877 and 1882. Koch stated that these postulates should be met before a causal relationship can be accepted between a particular bacterial parasite or disease agent and the disease in question:

  1. The agent must be shown to be present in every case of the disease by isolation in pure culture.
  2. The agent must not be found in cases of other disease.
  3. Once isolated, the agent must be capable of reproducing the disease in experimental
  4. The agent must be recovered from the experimental disease produced.

115 Hierarchy of evidence

Postulates 1 and 2 require complete specificity in a unique and unconfounded bacterial cause; 3 demands biological coherence; and 4 requires performance as predicted in experimental tests. Insistence on the invariable presence of the organism (postulate 1) conforms with Galileo’s original notion of necessary and sufficient cause. For the more recently recognized viruses and prions, which lack independent life and often specificity also, the generalizations of the postulates do not hold. Nor do they hold for diseases of complex etiology, which cause most of the burden of disease in many areas of the world.10,66,67,169,190 See also causal criteria.

HERD IMMUNITY The immunity of a group or community. The resistance of a group to invasion and spread of an infectious agent, based on the resistance to infection of a high proportion of individual members of the group. The resistance is a product of the number susceptible and the probability that those who are susceptible will come into contact with an infected person. Resistance of a population to invasion and spread of an infectious agent, based on the agent-specific immunity of a high proportion of the popu- lation. The proportion of the population required to be immune varies according to the agent, its transmission characteristics, the distribution of immunes and susceptibles, and other (e.g., environmental) factors.

HERDIMMUNITYTHRESHOLD Theproportionofimmunesinapopulation,abovewhich the incidence of the infection decreases.233 This can be mathematically expressed as

where H is the herd immunity threshold, R0 is the basic reproductive rate, r is the transmission parameter, and T is the total population.

HEREDITY The passing on of biological (including genetic) characteristics from one gen- eration to the next. Nonbiological traits and attributes may also be passed on from parents to offspring (e.g., religious and health beliefs). The transmission of characters and dispositions in the process of organic reproduction. Introduced in the biomedical sciences from the legal sphere, where it was used synonymous with inheritance and succession. 234,235

HERITABILITY The degree to which a trait is genetically determined, calculated by regression-correlation analyses among close relatives.

HETEROSCEDASTICITY Nonconstancy of the variance of a measure over the levels of the factors under study.

HEURISTIC METHOD A method of reasoning that relies on a combination of empiri- cal observations and unproven theories to produce a solution that may be correct and defensible but cannot be proven sound under the given conditions of application. The word, not always perfectly understood by users or audience, sounds more impressive than the method. In common parlance, rule of thumb.

HIBERNATION Survival of organisms (including arthropod vectors) during cold periods by reducing the metabolic rate.

HIERARCHICAL ANALYSIS See multilevel analysis.
HIERARCHICAL MODEL See multilevel model.
HIERARCHY OF EVIDENCE The quality of epidemiological evidence was appraised by

the Canadian Task Force on the Periodic Health Examination236 and the U.S. Preven- tive Services Task Force237 as an essential prerequisite to their recommendations about

“High-risk” preventive strategy 116

screening and preventive interventions. The classes of evidence that these groups used are as follows:

I: Evidence from at least one properly designed randomized controlled trial.

  • II-1:  Evidence from well-designed controlled trials without random allocation.
  • II-2:  Evidence from well-designed cohort or case-control analytic studies, preferably
    from more than one center or research group.
  • II-3:  Evidence obtained from multiple time series, with or without the intervention;
    dramatic results in uncontrolled experiments (e.g., first use of penicillin in the
    1940s) also are in this category.

III: Opinions of respected authorities, based on clinical experience, descriptive

studies, reports of expert committees, consensus conferences, etc.
It is not always possible to achieve complete scientific rigor; for example, randomized

controlled trials or cohort studies may be unethical or not feasible.
“HIGH-RISK” PREVENTIVE STRATEGY See strategy, “high-risk.”

INFERENCE A series of logical, empirical, and theoretical checks that causal rela- tions may or may not satisfy, described by Austin Bradford Hill (1897–1991)238 and elaborated by others, including Mervyn Susser.67 The considerations are often called “criteria,” even though Hill did not use the latter term. They are:

Consistency: The association is consistent when results are replicated in studies in different settings using different methods. Replicability and survivability.
Strength: This is defined by the size of the risk as measured by appropriate statistical estimates. The stronger, the more likely to be causal, although weak relationships may also be causal.
43 See also absolute risk difference; relative risk; risk difference.

Specificity: Present when a putative cause produces a specific effect, as hypothesized or predicted by background theory (e.g., exogenous estrogen usage is expected to show a relation to hormone-sensitive conditions but not to seat-belt use). The particularity with which one variable predicts the occurrence of another. Dose-response relationship: An increasing level of exposure (in amount and/or time) increases the risk. More generally, the relation of exposure to risk follows the expected theoretical pattern (e.g., morbidity and mortality follow a U-shaped relation to some vitamins).

Temporal relationship: Exposure always precedes the outcome. This is the only absolutely essential or necessary criterion of causality. See also time order. Biological plausibility: The association is coherent with firmly established knowledge on pathobiological processes. Exceptional caution is needed in this consideration: when the understanding of biological mechanisms is incomplete, implausible and speculative biological explanations will seem plausible and even coherent. See also plausibility.

Coherence: The association should be compatible with existing theory and knowledge. It may be theoretical and factual; biological, clinical, epidemiological, social, statistical, etc. See also coherence, epidemiological.
Experiment: The caused condition can be altered (e.g., prevented or ameliorated) by an appropriate experimental regimen that changes the putative effect.

See also association; causal criteria; causality; causation of disease, factors in; coherence; diseases of complex etiology; Evans’s postulates; Henle-Koch postulates; Mill’s canons; necessary cause; probability of causation.

HISTOGRAM A graphic representation of the frequency distribution of a variable. Rectangles are drawn in such a way that their bases lie on a linear scale representing different intervals, and their areas are proportional to the frequencies of the values within each of the intervals. See also bar chart.

HISTORICAL COHORT STUDY See cohort study, historical.

HISTORICAL CONTROL Control subject(s) for whom data were collected at a time preceding that at which the data are gathered on the group being studied. Because of differences in exposure, etc., use of historical controls can lead to bias in analysis.

HISTORY OF EPIDEMIOLOGICAL METHODS See epidemiological methods, history of.

HIV SEROCONCORDANT / -DISCORDANT Sexual partners having/not having the same HIV serological status.

HOGBENNUMBER A unique personal identifying number constructed by using a sequence of digits for birth date, sex, birthplace, and other identifiers. Suggested by the English mathematician Lancelot Hogben. Used in primary care epidemiology in some countries and usable in record linkage. See also identification number; soundex code.

HOLLERITH CARDS See punch card.

HOLOENDEMIC DISEASE A disease for which a high prevalent level of infection begins early in life and affects most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children. Malaria in many communities is a holoendemic disease.

HOLOMIANTIC INFECTION See common source epidemic.

HOMOSCEDASTICITY Constancy of the variance of a measure over the levels of the factors under study.

HOSPITAL-ACQUIRED INFECTION See nosocomial infection.

HOSPITAL DISCHARGE ABSTRACT SYSTEM Abstraction of minimum data set from hospital charts for the purpose of producing summary statistics about hospitalized patients. Examples include the Hospital Inpatient Enquiry (HIPE) and Professional Activity Study (PAS). The statistical tabulations commonly include length of stay by final diagnosis, surgical operations, specified hospital service (i.e., medical, surgical, gynecological, etc.) and also give outcomes such as “death” and “discharged alive from hospital.” This system cannot generally be used for epidemiological purposes as it is not possible to infer representativeness or to generalize; this is because the data usually lack a defined denominator and the same person may be counted more than once in the event of two or more hospital separations in the period of study. However, such data can be a fruitful source of cases for case-control studies of rare conditions.
The systematic use of summary statistics on the process and outcome of hospital care began in the nineteenth century, pioneered in England by Florence Nightingale (1820–1910) and in Vienna by Ignaz Semmelweis (1818–1865). Nightingale was the founder of modern nursing care and an accomplished statistician—a member of the Royal Statistical Society. She was also a confrere of William Farr, Edwin Chadwick, and other great nineteenth-century reformers. Her Notes on Hospitals (1859) discussed and illustrated the importance of statistical analysis of hospital activity. Semmelweis studied the outcome of obstetric care, demonstrating that puerperal sepsis was associated with attendance on women in labor by doctors who had come from the necropsy room to the labor room without washing their hands.

HOSPITAL EPIDEMIOLOGY The application of epidemiological reasoning, knowledge, and methods in hospitals (and, by extension, in other health care settings), in order to address a wide range of preventive issues and in particular to enhance the quality of patient care and the safety of health professionals. Intense efforts are always devoted to infection prevention and control and to the prevention of other adverse outcomes. As mentioned before, epidemiology is applied in many settings; for space reasons, this dictionary includes only a few illustrative examples of specialties. See also field epidemiology; nosocomial infection; primary care epidemiology.

HOSPITAL INPATIENT ENQUIRY (HIPE) Statistical tables of a 10% sample of hospital patients in England and Wales, showing class of hospital, diagnosis, length of stay, outcomes, etc.

HOSPITAL SEPARATION A term used in commentaries on hospital statistics to describe the departure of a patient from hospital without distinguishing whether the patient departed alive or dead. The distinction is unimportant insofar as the statistics of hospital activity, such as bed occupancy, are concerned.


  1. A person or other living animal, including birds and arthropods, that affords subsistence or lodgment to an infectious agent under natural conditions. Some protozoa and helminths pass successive stages in alternate hosts of different species. Hosts in which the parasite attains maturity or passes its sexual stage are primary or definitive hosts; those in which the parasite is in a larval or asexual state are secondary or intermediate hosts. A transport host is a carrier in which the organism remains alive but does not undergo development.52
  2. In an epidemiological context, the host may also be the population or group; biological, social, and behavioral characteristics of this group relevant to health are called “host factors.”

HOST, DEFINITIVE In parasitology, the host in which sexual maturation occurs. In malaria, the mosquito (invertebrate host).

HOST, INTERMEDIATE In parasitology, the host in which asexual forms of the parasite develop. In malaria, this is a human or other vertebrate mammal or bird (vertebrate host).

HOUSEHOLD One or more persons who occupy a dwelling (i.e., a place that provides shelter, cooking, washing, and sleeping facilities); this may or may not be a family. The term is also used to describe the dwelling unit in which the persons live.

HOUSEHOLD SAMPLE SURVEY A survey of persons in a sample of households. This, in many variations, is a favored method of gathering data for health-related and many other purposes. The households may be sampled in any of several ways—e.g., by cluster or use of random numbers in relation to numbered dwelling units. The survey may be conducted by interview, telephone survey, or self-completed responses to presented questions. The method is used in developing nations as well as in the industrial world.

HUMAN BLOOD INDEX Proportion of insect vectors found to contain human blood.

HUMAN DEVELOPMENT The process by which individuals, social groups, and populations achieve their potential level of health and well-being. Human development includes physical, biological, mental, emotional, educational, economic, social, and cultural components; some of these are expressed in the Human Development Index (HDI).

HUMAN DEVELOPMENT INDEX (HDI) A composite index combining indicators rep- resenting three dimensions—longevity (life expectancy at birth); knowledge (adult literacy rate and mean years of schooling); and income (real GDP per capita in purchasing-power-parity dollars) (source: World Bank). The validity of the HDI has been questioned because it attempts to express multiple complex variables on a unidimensional scale.

HUMAN ECOLOGY The study of human groups as influenced by environmental factors, including social and behavioral factors. A macrolevel, holistic approach to the study of human organization. See also ecology.

HUMAN GENOME EPIDEMIOLOGY NETWORK (HUGENET) A collaboration committed to assess the impact of human genome variation on population health and how genetic information can be used to improve health and prevent disease, including assessment of the role and quality of genetic tests for screening (www.cdc.gov/genom- ics/hugenet). See also genetic epidemiology; molecular epidemiology.

HUMAN IMMUNODEFICIENCY VIRUS (HIV) The pathogenic organism responsible for the acquired immunodeficiency syndrome (AIDS). Formerly known as the lymphad- enopathy virus (LAV), the name given by Montagnier et al., the original French discoverers, in 1983; it was also known as the human T-cell lymphotropic virus, type III (HTLV-III), the name given by Gallo et al. to the virus they reported in 1984. The ret- rovirus responsible for HIV disease, it is transmissible in blood, serum, semen, breast- feeding, body tissues, and other body fluids. The are two types: HIV-1 (responsible of the AIDS pandemic) and HIV-2; they compromise immune responses to organisms that are destroyed by a healthy immune system. The virus is immunologically unstable, but it produces antibodies that can be detected by Western blot and ELISA tests of blood, serum, semen, saliva, etc.

HUMAN IMMUNODEFICIENCY VIRUS (HIV) INFECTION The surveillance case defini- tion for HIV infection uses either laboratory, clinical, or other criteria. The laboratory criteria include positive screening test for HIV antibody and other laboratory evidence of HIV infection. Additional criteria of the presence and absence of HIV infection are defined for infants.

HYGIENE The principles and laws governing the preservation of health and their practi- cal application. Practices conducive to good health. The sum of the procedures and techniques that promote human development and harmonious adaptation to the indi- vidual’s milieu.

HYPERENDEMIC DISEASE A disease that is constantly present at a high incidence and/ or prevalence and affects most or all age groups equally.

HYPERGEOMETRIC DISTRIBUTION The exact probability distribution of the frequencies in a two-by-two contingency table, conditional on the marginal frequencies being fixed at their observed levels.

HYPNOZOITE Dormant form of malaria parasites found in liver cells. After sporozoites (inoculated by the mosquito) invade liver cells, some sporozoites develop into dormant forms (the hypnozoites), which do not cause symptoms. Hypnozoites can become acti- vated months or years after the initial infection, producing a relapse.


1. A supposition, arrived at from observation or reflection, that leads to refutable predictions.

2. Any conjecture cast in a form that will allow it to be tested and refuted. See also null hypothesis.

HYPOTHETICO-DEDUCTIVE METHOD Karl Popper’s language (following Hume) for deductive logic as applied to scientific research. For Popper, science is that which is testable.239 Therefore a hypothesis must be stated a priori in order to test its survivability by efforts to reject it. Popper’s underlying assumption is that no hypothesis can be truly verified and proved; at best, it can be corroborated. Many scientists reject Popper’s assumption of nonverifiability. In order to verify ideas and hypotheses, scientists are often obliged to argue by induction. But that does not preclude resort to the hypothetico-deductive method as a testing procedure.71 See also inductive logic; logic.

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