COVID-19 Glossary

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An immune system that isn’t functioning correctly. A person can be immunocompromised by diseases like AIDS or taking some anti-cancer drugs, but also by losing sleep, not drinking enough water and eating poorly. Pregnant women aren’t considered immunocompromised, but changes to their immune system can make them more susceptible to some diseases.


People with immune systems that are (usually artificially) weakened. People with organ transplants take immunosuppressants to stop their immune systems from attacking the organs. Immunosuppressed people are also immunocompromised.

in vitro

A lab test done on cells, not living things. We do in vitro tests on drugs, diseases and chemicals to see how they impact human cells.

in vivo

A lab test on a living organism, whether that’s a plant or an animal or something else. Drugs are usually tested in vivo to make sure they’re safe and that they work.

incubation period

The amount of time it takes for an infected person to start showing symptoms. Most people develop COVID-19 symptoms by day 12, but some people will take longer.

index case

The first documented case of an infectious disease or genetically transmitted condition or mutation in a population, region, or family. It may also refer to an individual who has a disease, condition, or mutation that is the first one identified in a population. This second sense is synonymous with index patient. A related term is patient zero, “a person identified as the first to become infected with an illness or disease in an outbreak.” Patient zero is especially used to refer to a person documented as being the first known case of a communicable disease in a particular population or region.


Producing or capable of producing infection, containing pathogenic agents which may be transmitted. 

An ailment such as food poisoning is infectious, it is capable of producing infection, but it is not contagious. The coronavirus, on the other hand, is both contagious and _infectious. Anything that is contagious is automatically also infectious, but the reverse is not true. Both words are frequently used in a figurative manner.


When a person who is showing symptoms of a disease separates themselves from other people to prevent spreading the disease to others.