An epidemiologist’s job is to study the cause, effect, distribution, pattern, and control of population health and disease. The methods used by an epidemiologist are important in HIV quality management and improvement; however, the terms used are often confusing. This article seeks to describe many of the ideas, measures, and vocabulary used by these professionals. This information should not be seen as a comprehensive list of epidemiological terms because it is far from comprehensive. Instead, this article touches upon those terms most frequently seen in epidemiological data or reports.

Infectious diseases are those caused by agents that replicate in their host and cause disease. The most common agents are bacterial, viral, or fungal; however, protozoa and helminths (worms) are also considered to be infectious agents. When an infection can pass from one person or animal to another, it is a communicable disease.

Chronic diseases are those lasting at least three months (i.e., type 2 diabetes and obesity).

The incubation (infectious) or latency (chronic) period of a disease is the period of time between exposure and disease onset when only subclinical changes occur.

Pathogenicity is the proportion of individuals infected following exposure to an infectious agent. Similar measures are attack rate, the frequency of infection among those exposed, and secondary attack rate, the frequency of infection among those who were in contact with the infected.

Virulence is the proportion of individuals who become severely ill or die after becoming infected.

An epidemic, or outbreak, occurs when disease or injury occurs at a higher than expected rate in a given area and varies by condition. A flu outbreak may not be declared until thousands are diagnosed; whereas, one small pox diagnosis is an outbreak. A pandemic is an epidemic affecting a large proportion of the population over a wide geographic area.

Active immunity is resistance to an infection developed in response to an antigen. This can follow vaccination against, or infection with, an infectious agent (e.g., measles). In comparison, passive immunity is acquired from an external source such as antibodies passed from mother to infant (transplacentally or in breast milk) or by administration of immune globulin.

Herd immunity is group resistance to an infection based on the proportion of group members resistant to infection. For instance, pertussis vaccination among a high proportion of residents helps to prevent outbreaks of the disease among those too young to have been vaccinated.

This article barely scratches the surface of the epidemiological vernacular. If you encounter a term that you need to understand better, you can turn to the Centers for Disease Control and Prevention (CDC) to look it up. The CDC’s epidemiological glossary can be found at http://bit.ly/2ove1rj.