cdc glossary

disease prep

Age-Adjusted Mortality Rate

A mortality rate statistically modified to eliminate the effect of different age distributions in different populations.

Age-Specific Mortality Rate

A mortality rate limited to a specific age group.

Agent

A factor—such as a microorganism, chemical, or radiation—whose presence, excessive presence, or absence is essential for disease occurrence.

Analytic Epidemiology

The branch of epidemiology that searches for health-related causes and effects by using comparison groups to test causal hypotheses.

Analytic Study

A comparative study designed to identify and quantify associations, test hypotheses, and identify causes (e.g., cohort or case-control study).

Association

A statistical relationship between two variables or events.

Attack Rate

An incidence rate applied to a narrowly defined population during an outbreak.

Attributable Proportion

The proportion of disease among the exposed that is attributable to the exposure.

Bar Chart

A visual display that represents the size of different categories with bars.

Biologic Transmission

Indirect vector-borne transmission in which the agent undergoes changes within the vector before transmission to the host.

Box Plot

A visual summary of data using a ‘box and whiskers’ format showing minimum, maximum, interquartile range, and median values.

Case

A countable instance of a particular disease, condition, or event in a population.

Case Definition

Standard criteria for classifying whether a person has a particular disease or health condition based on clinical and epidemiologic features.

Case-Control Study

An observational analytic study in which subjects are selected based on disease status (cases vs. controls) and prior exposures are compared.

Case-Fatality Rate

The proportion of persons with a specified condition who die from that condition.

Carrier

A person or animal without apparent disease who harbors a specific infectious agent and can transmit it to others.

Cause Of Disease

A factor that directly influences the occurrence of disease and whose removal reduces disease incidence.

Cause-Specific Mortality Rate

The mortality rate attributed to a specific cause of death.

Census Population

(See Population) The total number of inhabitants in a given area as counted at a specific time.

Chain Of Infection

The process by which an infectious agent passes from a reservoir to a susceptible host through a portal of exit, mode of transmission, and portal of entry.

Class Interval

The span of values grouped into one category in a frequency distribution.

Common Source Outbreak

An outbreak that results from a group being exposed to a common harmful influence.

Confidence Interval

A range of values that likely contains the true value of a parameter with a stated probability.

Confidence Limit

The upper or lower boundary of a confidence interval.

Contact

Exposure to a source of infection, or a person who has been so exposed.

Contagious

Capable of being transmitted from one person to another by contact or close proximity.

Contingency Table

A table used to display the frequency distribution of variables and analyze their relationships (cross-tabulation).

Control (in Case-Control Study)

A person without the disease or condition used as a comparison in a case-control study.

Cohort Study

An observational analytic study in which subjects are classified according to exposure status and followed over time to ascertain outcomes.

Crude Mortality Rate

The mortality rate from all causes in an entire population.

Cumulative Frequency

The number or proportion of observations with a particular value or lower in a frequency distribution.

Cumulative Frequency Curve

A plot of cumulative frequency against class intervals.

Demographic Information

Descriptive traits of a population such as age, sex, race, and occupation.

Dependent Variable

The outcome of interest in statistical analysis that is influenced by independent variables.

Determinant

Any definable factor that affects a change in a health condition.

Direct Transmission

Immediate transfer of an infectious agent from reservoir to susceptible host via direct contact or droplet spread.

Disease

(General term) A disorder or illness affecting an organism’s structure or function.

Dot Plot

A visual display of individual data points for a non-continuous variable.

Droplet Nuclei

Residue of dried respiratory droplets that can remain suspended in air and be inhaled over long distances.

Droplet Spread

Direct transmission via large respiratory droplets produced by sneezing, coughing, or talking.

Epidemic

An occurrence of more cases of disease than expected in a given area or population over a period of time.

Epidemic Curve

A histogram that displays the number of outbreak cases by time of onset.

Epidemic Period

The time span during which the number of cases exceeds the expected level.

Endemic Disease

The constant presence of a disease or infectious agent within a geographic area or population.

Environmental Factor

An extrinsic element—such as climate or sanitation—that affects disease occurrence or progression.

Epidemiologic Evaluation

(See Evaluation) Assessment of relevance, effectiveness, efficiency, and impact of health programs.

Evaluation

A process to assess the relevance, effectiveness, and impact of public health activities or programs.

Experimental Study

A study in which the investigator assigns exposures (e.g., in a clinical or community trial) and observes outcomes.

Exposed Group

A group whose members have experienced a suspected causal factor or determinant.

Exposure

Contact with a factor that is presumed to influence health.

Frequency Distribution

A summary of how often each value or category of a variable occurs, often displayed in a table.

Frequency Polygon

A graph that connects the mid-points of the tops of bars in a histogram to show frequency distribution.

Geometric Mean

The average of data measured on a logarithmic scale, calculated as the nth root of the product of n values.

Graph

A visual representation of quantitative data using coordinates, axes, and symbols.

Health Indicator

A measure that reflects the health status of a population, such as the infant mortality rate.

Health Information System

An organized collection of health statistics used to assess health status and services.

High-Risk Group

A subgroup of the population with an elevated risk of disease or health event.

Histogram

A graphic representation of the frequency distribution of a continuous variable using adjoining bars.

Host

An organism that can be infected by an infectious agent.

Host Factor

An intrinsic characteristic—such as age, sex, genetics—that influences an individual’s susceptibility or response to an agent.

Hyperendemic Disease

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

Hypothesis

A refutable conjecture predicting a relationship between exposure and outcome, often tested statistically.

Hypothesis, Alternative

The proposition that exposure is associated with disease (difference exists).

Hypothesis, Null

The proposition that there is no association between exposure and disease.

Immunity, Active

Resistance developed by the host after exposure to an antigen through infection or vaccination.

Immunity, Herd

Resistance of a group to disease spread due to a high proportion of immune individuals.

Immunity, Passive

Immunity acquired through antibodies produced in another host and transferred naturally or artificially.

Incidence Rate

The frequency with which new events (e.g., illnesses) occur in a population during a specified time period.

Incubation Period

The time between exposure to an infectious agent and onset of symptoms.

Independent Variable

A factor believed to influence or predict changes in the dependent variable.

Indirect Transmission

Transfer of an agent from reservoir to host via airborne particles, vectors, or vehicles.

Infant Mortality Rate

Deaths among children under one year of age per 1,000 live births.

Infectivity

The proportion of exposed individuals who become infected.

Interquartile Range

The range between the 25th and 75th percentiles that contains the middle 50% of values.

Latency Period

The time between exposure and onset of symptoms for chronic diseases.

Mean (Arithmetic)

The average of a set of values obtained by dividing the sum of the values by their number.

Median

The value that divides a data set into two equal halves.

Measure Of Association

A quantified relationship between exposure and disease, such as risk ratio or odds ratio.

Measure Of Central Location

A value representing the center of a data set (mean, median, or mode).

Measure Of Dispersion

A statistic indicating the spread of data around a central value (e.g., variance, standard deviation).

Medical Surveillance

Monitoring of exposed individuals for early detection of disease symptoms.

Midrange

The midpoint of a data set, calculated as (minimum + maximum) / 2.

Mode

The most frequently occurring value in a data set.

Morbidity

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

Mortality

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

Mortality Rate

The frequency of death in a defined population during a specified time interval.

Mortality Rate, Neonatal

Deaths from birth to under 28 days of age per 1,000 live births.

Mortality Rate, Postneonatal

Deaths from 28 days to under one year per 1,000 live births.

Natural History Of Disease

The progression of a disease process in an individual over time without intervention.

Necessary Cause

A causal factor whose presence is required for the disease to occur.

Nominal Scale

A categorical measurement scale without inherent order (e.g., blood type).

Normal Curve

A bell-shaped curve that results from graphing a normal distribution.

Normal Distribution

A symmetrical distribution where mean, median, and mode are equal and located at the center.

Ordinal Scale

A categorical scale with inherent order but without equal intervals (e.g., stage I, II, III).

Outbreak

Synonym for epidemic, often used for localized increase in disease cases.

Pandemic

An epidemic occurring over several countries or continents and affecting a large proportion of the population.

Pathogenicity

The proportion of infected individuals who develop clinically apparent disease.

Person-Time Rate

An incidence rate that uses person-time as the denominator.

Pie Chart

A circular chart in which slice sizes represent the proportion of categories.

Population

The total number of inhabitants in a specified area or group.