pathogen

An agent that causes disease, especially a virus, bacterium, or fungus.

Any microorganism which causes disease; a pathogenic organism; an infectious microorganism; a bacterium, virus, or other agent which can cause disease by infection; — opposed to zymogene. The spelling pathogene is now archaic.

Any organism or substance, especially a microorganism, capable of causing disease, such as bacteria, viruses, protozoa or fungi. Microorganisms are not considered to be pathogenic until they have reached a population size that is large enough to cause disease.

any disease-producing agent (especially a virus or bacterium or other microorganism)