Basic terms in Disease Epidemiology: Infectious diseases

Let’s start with the word disease:

Disease literally means dis+ ease= not at ease. Of course, this is overly simplistic, but is efficient at communicating the essence of the matter. It is important to remember that the definition of what constitutes a ‘disease’ has evolved over time. The term also varies with culture.

So what are the main types of diseases?

There are numerous classifications of diseases, but for now we will focus on the simplest: Infectious (Communicable) and Non-infectious (Non-communicable) diseases.

A. Infectious (Communicable) disease: A disease caused by an infection

Infection: The invasion of a host by disease causing agents, their multiplication, and the reaction of the host tissues to these organisms and their toxins.

Infectious agent: Anything capable of causing an infection: viruses, bacteria, prions, protozoa, nematodes, arthropods, etc.

Communicable disease: Although considered to be synonymous infectious disease, a communicable disease is one that can be communicated (transferred) from an infected organism to an uninfected (and susceptible) organism. The transmission of infection may occur through direct or indirect means.

Direct transmission: Through touch, inhalation, sexual contact, contact with infected discharges etc.

Indirect transmission: Through vectors, for example.

Vector: Something (living) that carries and transfers an infectious agent into another living organism. Example: Mosquito

Fomite: An inanimate object or substance that carries (and therefore transfers) an infectious agent from one organism to another. Example: Doorknob, Bedsheets

Contagious disease: A special type of communicable disease that easily spreads from one to another. The distinction between communicable and contagious is important because those with contagious diseases are usually subjected to isolation or quarantine. Example: Measles

Isolation: The separation of an infected organism, after the development of infection, from other (susceptible) organisms , for the period of communicability.

The purpose is to protect others from getting infected, and limit the spread of disease.

Quarantine: The separation of an organism from other (susceptible) organisms for the duration of incubation period.

The intention is the same as in isolation, but there is one important difference between the two: here we are not sure if the organism actually is infected. Since we are not sure, we segregate the organism for as long as it would take for the disease to ordinarily manifest itself after infection (incubation period). Once the incubation period is over, we definitively know whether the organism was infected or not. this allows appropriate responses to be initiated.

Period of communicability: The length of time, from acquiring the infection, during which the infection can be communicated/ transmitted to another uninfected organism.

Incubation period: The time it takes for the first clinical symptoms to appear after infection.

Host: The organism harboring the infectious agent (usually a parasite)

Definitive Host: The host in which the parasite achieves sexual maturity

Intermediate Host: The host(s) in which the parasite develops but does not attain sexual maturity

Note: When I say ‘organism’, I am including humans

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One thought on “Basic terms in Disease Epidemiology: Infectious diseases

  1. Pingback: Basic Terms in Disease Epidemiology: Non-Communicable Diseases | communitymedicine4asses

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