Disease Detectives Cheat Sheet

Absolute Risk

The absolute or calculated probability of an event in a population under study.

Absolute Risk Difference

Risk in Exposed Population - Risk in Unexposed Population

Agent

What causes a disease. For example, a microorganism, chemical substance, or form of radiation.

Active Immunity

Resistance developed in response to stimulus by an antigen (infecting agent or vaccine) and usually characterized by the presence of antibody produced by the host.

Attack Rate

Number of incidences/Total population

Bias (Systematic Error)

Deviation of results or inferences from the truth, or processes based in bad study design leading to such deviation.

Biologic Transmission

The indirect vector-borne transmission of an infectious agent in which the agent undergoes biologic changes within the vector before being transmitted to a new host.

Causality

The relating of causes to the effects they produce.

Clinical Outcomes

Measures patient health or well being.

Co-interventions

Interventions other than the treatment under study that may have been applied differently to the study and control groups.

Co-morbidity

Coexistence of a disease or diseases in a study participant in addition to the index condition that is the subject of study.

Carrier

A person or animal without apparent disease who harbors a specific infectious agent and is capable of transmitting the agent to others.

Case

In epidemiology, a countable instance in the population or study group of a particular disease, health disorder, or condition under investigation.

Case Definition

A set of standard criteria for deciding whether a person has a particular disease or health-related condition, by specifying clinical criteria and limitations on time, place, and person.

Case-Fatality Rate

The proportion of persons with a particular condition (cases) who die from that condition. The denominator is the number of incident cases; the numerator is the number of cause-specific deaths among those cases.

Chain of Infection

A process that begins when an agent leaves its reservoir or host through a portal of exit, and is conveyed by some mode of transmission, then enters through an appropriate portal of entry to infect a susceptible host.

Cluster

An aggregation of cases of a disease or other health-related condition, particularly cancer and birth defects, which are closely grouped in time and place.

Cohort

A well-defined group of people who have had a common experience or exposure, who are then followed up for the incidence of new diseases or events. A type of observational analytic study

Common Source Outbreak

An outbreak that results from a group of persons being exposed to a common noxious influence, such as an infectious agent or toxin.

Determinant

Any definable factor that affects a change in a health condition or other characteristic.

Direct Transmission

The immediate transfer of an agent from a reservoir to a susceptible host by direct contact or droplet spread.

Distribution

In epidemiology, the frequency and pattern of health-related characteristics and events in a population.

Effectiveness

A measure of the benefit resulting from an intervention administered under usual conditions of clinical care for a particular group of patients.

Etiology

The study of the cause or origin of a disease.

Exclusion Criteria

Stated conditions which preclude entrance of candidates into an investigation even if they meet the inclusion criteria.

Environmental Factor

An extrinsic factor (geology, climate, insects, sanitation, health services, etc.) which affects the agent and the opportunity for exposure.

Epidemic

The occurrence of more cases of disease than expected in a given area or among a specific group of people over a particular period of time.

Epidemic Curve

A histogram that shows the course of a disease outbreak or epidemic by plotting the number of cases by time of onset.

Epidemic Period

A time period when the number of cases of disease reported is greater than expected.

Epidemiologic Triad

The traditional model of infectious disease causation. includes three components: an external agent, a susceptible host, and an environment that brings the host and agent together, so that disease occurs.

Histogram

A graphic representation of the frequency distribution of a continuous variable.

Fomite

Any nonliving object or substance capable of carrying infectious organisms, such as germs or parasites, and hence transferring them from one individual to another.

Host

A person or other living organism that can be infected by an infectious agent under natural conditions.

Hyperendemic Disease

A disease that is constantly present at a high incidence and/or prevalence rate.

Herd Immunity

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.

Incidence

The rate of new cases of illness commencing during a specified time period in a given population.

Interviewer Bias

Systematic error due to interviewer's subconscious or conscious gathering of selective data.

Incidence Rate

A measure of the frequency with which an event, such as a new case of illness, occurs in a population over a period of time. The denominator is the population at risk; the numerator is the number of new cases occurring during a given time period.

Incubation Period

A period of subclinical or inapparent pathologic changes following exposure, ending with the onset of symptoms of infectious disease.

Independent Variable

An exposure, risk factor, or other characteristic being observed or measured that is hypothesized to influence an event or manifestation (the dependent variable).

Indirect Transmission

The transmission of an agent carried from a reservoir to a susceptible host by suspended air particles or by animate (vector) or inanimate (vehicle) intermediaries.

Lead-Time Bias

Overestimation of survival because of earlier diagnosis—time of death does not change, just time of diagnosis.

Morbidity

Any departure, subjective or objective, from a state of physiological or psychological well-being.

Mortality Rate

A measure of the frequency of occurrence of death in a defined population during a specified interval of time.

Number Needed to Treat (NNT)

The number of patients with a particular condition who must receive an intervention to prevent the occurrence of one adverse outcome

Odds

A ratio between two probabilities—the probability of an event to a non-event.

Odds Ratio (OR)

The odds of the experimental group showing positive (or negative) effects of an intervention or exposure, in comparison to the control group.

Observational Study

Epidemiological study in situations where nature is allowed to take its course. Changes or differences in one characteristic are studied in relation to changes or differences in others, without the intervention of the investigator.

Outbreak

Synonymous with epidemic. Sometimes the preferred word, as it may escape sensationalism associated with the word epidemic. Alternatively, a localized as opposed to generalized epidemic.

Prevalence

The proportion of persons with a particular disease within a given population at a given time.

Prognosis

The possible outcomes of a disease or condition and the likelihood that each one will occur.

Passive Immunity

Immunity conferred by an antibody produced in another host and acquired naturally by an infant from its mother or artificially by administration of an antibody-containing preparation (antiserum or immune globulin).

Pandemic

An epidemic occurring over a very wide area (several countries or continents) and usually affecting a large proportion of the population.

Pathogenicity

The proportion of persons infected, after exposure to a causative agent, who then develop clinical disease.

Randomized Controlled Trial

An experimental comparison study in which participants are allocated via a randomization mechanism to either an intervention/treatment group or a control /placebo group, then followed over time and assessed for the outcomes of interest. Participants have an equal chance of being allocated to either group.

Recall Bias

Systematic error due to the differences in accuracy or completeness of recall to memory of past events or experiences.

Relative Risk (RR)

The ratio of the probability of developing, in a specified period of time, an outcome among those receiving the treatment of interest or exposed to a risk factor, compared with the probability of developing the outcome if the risk factor or intervention is not present.

Reproducibility (Repeatability, Reliability)

The results of a test or measure are identical or closely similar each time it is conducted.

Risk Factor

Patient characteristics or factors associated with an increased probability of developing a condition or disease in the first place. Compare with prognostic factors.

Relative Risk Reduction (RRR)

The extent to which a treatment reduces a risk, in comparison with patients not receiving the treatment of interest.

Relative Risk

A comparison of the risk of some health-related event such as disease or death in two groups.

Representative Sample

A sample whose characteristics correspond to those of the original population or reference population.

Reservoir

The habitat in which an infectious agent normally lives, grows and multiplies; reservoirs include human reservoirs, animals reservoirs, and environmental reservoirs.

Risk

The probability that an event will occur, e.g. that an individual will become ill or die within a stated period of time or age.

Risk Ratio

A comparison of the risk of some health-related event such as disease or death in two groups.

Surveillance

The ongoing systematic collection, recording, analysis, interpretation, and dissemination of data reflecting the current health status of a community or population

Selection Bias

A bias in assignment or a confounding variable that arises from study design rather than by chance. These can occur when the study and control groups are chosen so that they differ from each other by one or more factors that may affect the outcome of the study.

Secondary Attack Rate

A measure of the frequency of new cases of a disease among the contacts of known cases.

Sporadic

A disease that occurs infrequently and irregularly.

Vector

Any agent (animal, or microorganism) that carries and transmits an infectious pathogen into another living organism.

Validity

The degree to which the results of a study are likely to be true, believable and free of bias. This is entirely independent of the precision of the results and does not predict the results to your patients.

Vehicle

An inanimate intermediary in the indirect transmission of an agent that carries the agent from a reservoir to a susceptible host.

Virulence

The proportion of persons with clinical disease, who after becoming infected, become severely ill or die.

Zoonoses

An infectious disease that is transmissible under normal conditions from animals to humans.