The habitat where a pathogen normally resides and reproduces. This may include hosts such as humans and animals, and the environment (e.g., bodies of water).

Severe illness

An individual experiencing severe clinical symptom(s), potentially requiring hospitalization.

Stochastic eradication/stochastic elimination (epidemic fade-out)

When a disease becomes extinct at random (by chance) in a geographic location even if Reproduction Number (R or R0) is greater than 1. This can take place in a small population or when the disease of interest is rare. Stochastic eradication/elimination (epidemic fade-out) may be modeled in stochastic models, not deterministic models.


An uninfected individual/population without sufficient immunity against a pathogen to prevent infection and is at risk of developing the disease with sufficient exposure (e.g., a Close contact).


The process of a pathogen being transferred from an infected individual to a susceptible individual through direct or indirect contact, airborne particles, or vectors (e.g., mosquitoes).


A biological preparation that provides immunity to a specific disease. Vaccines often contain weakened or inactivated forms of the pathogen, its toxins, or surface proteins that trigger an immune response in the body, producing antibodies and memory cells.


An organism that transmits a pathogen from one host to another, often an arthropod such as a mosquito, tick, or flea.