Many of these definitions come from the Center for Disease Control course:

Principles of Epidemiology in Public Health Practice.

Many more additional terms are defined at the CDC source.


antibody—chemicals made by B lymphocytes that can bind to antigens.

antigen—any type of molecular structure that lymphocytes recognize as foreign to the body and that can trigger an immune response.

applied epidemiology—the field of epidemiology that addresses public health issues, such as communicable diseases or the environmental effects on cancer rates.


B cell—a lymphocyte that produces antibodies that bind to an antigen.

bacteria— one-celled, prokaryotic organisms which are involved in fermentation, nitrogen fixation, and infectious disease.

bacillus—rod-shaped bacteria.

bar chart—a graph in which each data value is represented by a bar or column.

bias—a certain tendency or inclination that prevents unprejudiced consideration of a question or decision that can lead to inaccurate conclusion.

bimodal—having two data peaks.

biologic transmission—transfer of an infectious agent by a vector; the vector is usually a key part of the agent life cycle and harbors the agent before it is transferred to the host.


carrier—a person or animal that bears an infectious disease agent and can transmit it to other, but usually does not demonstrate any signs of the disease.

case-an instance of a disease or injury.

case definition—in epidemiology, a description of specific conditions and details that should be met in order for the person to be identified as having a particular disease or other health condition.

case-fatality rate—the proportion of people with a particular condition who die from that condition; the number of deaths divided by the number of persons having the condition x 100.

chain of infection—the movement of an infectious agent from one host by way of a transmission process to infect another susceptible host.

clinical disease—a disease that is at a state in which it shows its symptoms.

clinical trial—an experimental study that tests a variable and collects data from individual persons and follows the person’s health condition to determine the effects of the variable tested.

coccus—round-shaped bacteria.

common-source outbreak—an outbreak that occurs from persons being exposed to the same harmful effect, such as the same infectious agent or toxin.

comparison group—a group used to compare results or data obtained in the experimental group.

contact—a person who has been exposed to a source of infection.

contact, direct—exposure or transmission of an infectious agent through touching; includes human-to-human contact and touching an infected animal, soil, or vegetation.

contagious—capable of being transmitted from one person to another by contact or close proximity.

control—in a case-control study, a member of the group of persons without the health problem under study.


definitive host—the host that harbors the adult stage of a parasite.

demographic information—personal characteristics of a person or group, for example, age, sex, race/ethnicity, or occupation; used in descriptive epidemiology to identify patients or populations .

dendrogram—a branching chart that shows the evolutionary lines or genetic relatedness of organisms.

descriptive epidemiology—the field of epidemiology that organizes and summarizes data regarding persons who are affected by a disease or conditions, time of onset, and location.

direct transmission—immediate transfer of a pathogen from one site or organism to a host by direct contact or droplet spread.

disease—a condition in which the body cannot function normally due to infection by a pathogenic agent, genetic condition, nutritional deficiency, or an illness of an affected body organ.

distribution—the frequency and pattern of health-related characteristics and events in a population.


efficacy—in epidemiology, the ability of a medical intervention or program to produce expected results under ideal conditions.

efficiency—in epidemiology, the effectiveness of a medical intervention or program to produce results with minimum time and resources.

endemic—a constant presence of a health condition within an area or population.

environmental factor—a factor in the environment that affects a pathogen and the opportunities for exposure to it. Examples include geology, climate, insects, sanitation, or quality of health services.

epidemic—numbers of disease cases, injuries, or other health conditions greater than expected in an area or population during a particular period.

epidemic curve—a graph that plots the course of an outbreak by time of onset (epi-curve).

epidemic period—the time span of an outbreak or epidemic.

epidemic modeling—methods which categorize individuals in a population according to their infection status in order to predict the spread of disease.

epidemiologic triad—in epidemiology, the model of infectious disease consisting of three components: an infectious agent, a susceptible host, and an environment that allows the two to come together.

epidemiology—the branch of medicine that studies patterns and causes of health-related conditions and uses those studies and results to control a health problem.

frequency—the amount or number of occurrences of a characteristic or health outcome among a population.

frequency distribution—a summary of the values or categories of a variable; usually displayed in a two-column table with the individual values or categories in the left column and the number of observations in the right column


health—the state of complete physical, mental, and social well-being.

helminth—worm-like parasites.

herd immunityresistance to an infectious agent by an entire group or community; a large number of immune persons in a community reduces the likelihood that an infected person will come into contact with a susceptible person among the population.

histogram—a graph using bars or columns to represent values in a statistical study; in epidemiology, time is usually plotted on the x-axis with number of cases on the y-axis.

hypothesis—an idea or explanation for something that is based on known facts but has not yet been proven.


immune system—a body system that protects against disease and foreign substances, destroys infected cells, and removes cellular debris.

Immunity, active—resistance to disease acquired by either exposure to a pathogen or by receiving a vaccine against a pathogen.

immunity, passive—immunity resulting from an antibody produced in another host, such as antibodies received by an infant from its mother, or vaccination.

immunology—the study of the immune system and the processes of the immune response against disease.

incidence—a measure of the frequency of new cases of disease, injury, or other health condition in a population during a specific time period.

incidence proportion—the number of new cases of illness, injury, or other health condition divided by the size of the population at the start of the time period.)

incidence rate—the probability of developing a specific disease during a given period of time; the number of new cases over a time period divided by the average population of the time period.

incubation period—the time period from exposure to an infectious agent to the onset of symptoms of disease.

independent variable—a variable being observed or measured that is hypothesized to have an effect on a dependent variable.

index case—the first case of a particular health condition of which health authorities are aware.

indirect transmission—transfer of an infectious agent by air, by object, or by a vector.

infection—the invasion of body tissues by an infectious organism; does not have to cause disease conditions.

infectious—a disease that is likely to be transmitted to organisms through the environment.

infectivity—the ability of an infectious organism to cause disease.

isolation—the separation of infected persons from the rest of a population to prevent transmission. (Quarantine is the separation of well persons who may have been exposed to the disease agent.)


latent—when a virus is capable of lying dormant within host cells for a period of time and becoming active in recurring episodes of symptomatic disease. 

line listing—in epidemiology, a spreadsheet list of cases or patients with information about each patient, including gender, age, and date of onset of symptoms.

longitudinal study—a study performed over a period of time so that effects of time are measured as a variable on the conditions.


microorganism—an organism too small to be seen without a microscope.



mortality rate—the rate or frequency of death in a population during a specific time period.


NIH—National Institutes of Health—an agency of the United States Department of Health and Human Services that is the primary agency of the US government responsible for biomedical and health-related research.

NIAID—National Institute of Allergy and Infectious Diseases—the branch of the National Institutes of Health that researches in order to understand, treat, and prevent infectious, immunologic, and allergic diseases.

outbreak—the sudden rise in the occurrence of a disease or injury in a given area during a given time period; generally considered to be more localized than an epidemic.

outbreak, common-source—an outbreak that occurs from persons being exposed to the same harmful effect, such as the same infectious agent or toxin.

outbreak, point-source—a type of source for an outbreak in which the exposure period is brief so that all cases of infection occur within one incubation period.

outbreak, propagated—an outbreak that spreads from person-to-person; not a common source outbreak.

outlier—a value that is markedly different from other values in a sample.

pandemic—an epidemic that occurs over a widespread area and affects a large proportion of the population.

pathogen—any disease-causing agent.

phagocytosis—the process of engulfing a smaller cell, microorganism, or foreign particle.

point-source outbreak—a type of source for an outbreak in which the exposure period is brief so that all cases of infection occur within one incubation period.

population—the number of persons in a geographic area or the number of persons in a particular group, such as the number of people who attend a certain school.

portal of entry—the pathway by which an infectious agent can enter its host. For example, the influenza virus’s portal of entry is host’s respiratory tract.

portal of exit—the pathway by which an infectious agent can leave its host. For example, the influenza virus’s portal of exit is the host’s respiratory tract (coughing or sneezing).

prevalence—the number of cases or events in a given population.


quarantine—a period of time in which a person or animal suspected of having an infectious disease is kept away from other people or animals; the separation of potentially exposed persons.


random sample—a small group of persons chosen by chance and not according to a plan or pattern, each one has the same probability of be selected.

reservoir—in epidemiology, the location in which an infectious agent normally lives; includes humans, animals, plants, or the environment.

resistance—the ability to withstand disease.


seasonality—change in status that occurs during a regular seasonal pattern. Examples include respiratory tract viral infections, flus, E. coli infections in Argentina, measles in Niger.

SIR model—mathematical model that analyzes and displays the spread of disease through a population using identified groups within the population: S for Susceptible, I for Infected, and R for Recovered/Removed.

source (of infection)—the person, animal, object, or substance from which an infectious agent is transmitted to a host.

transmission, airborne—transfer of an infectious agent through the air; an indirect transmission.

transmission, biologic—transfer of an infectious agent by a vector; the vector is usually a key part of the agent life cycle and harbors the agent before it is transferred to the host; an indirect transmission

transmission, direct—transfer of an infectious agent to a host by direct contact

transmission, indirect—transfer of an infectious agent by air, by object, or by a vector.

transmission, mechanical—transfer of an agent by a vector that is not part of the biologic life cycle of the infectious agent.

transmission, vectorborne—transfer of an infectious agent by a vector such as a tick, mosquito, or flea; an indirect transmission.

transmission, vehicleborne—transfer of an agent by an inanimate object; includes food or water.

vaccine—an antigen containing substance that stimulates the production of antibodies.

virology—the study of viruses.

virus—an infectious agent that multiplies only within the cells of living hosts; composed of RNA or DNA and a protein coat; more complex viruses also have a surrounding envelope.


x-axis—the horizontal axis on a line or bar graph.


y-axis—the vertical a on a line or bar graph.


zoonosis—an infectious disease transmitted from animals to humans.