A disease causing agent.

Virus, bacterium, parasitic protozoan, or other microorganisms that cause infectious disease by invading the body of an organism know as the host. Note that infection is not synonymous with disease because infection does not always lead to injury of the host.

(Pathogenic microorganisms) Microorganisms that can cause disease when they infect a host.

Microorganisms that can cause disease in other organisms or in humans, animals and plants (e.g., bacteria, viruses, or parasites) found in sewage, in runoff from farms or rural areas populated with domestic and wild animals, and in water used for swimming. Fish and shellfish contaminated by pathogens, or the contaminated water itself, can cause serious illness.