Infectious Agent

A pathogen or infectious agent is a biological agent that causes disease or illness to its host. The term is most often used for agents that disrupt the normal physiology of a multicellular animal or plant. However, pathogens can infect unicellular organisms from all of the biological kingdoms. The term pathogen is derived from the Greek παθογένεια, "that which produces suffering." There are several substrates and pathways where by pathogens can invade a host; the principal pathways have different episodic time frames, but soil contamination has the longest or most persistent potential for harboring a pathogen.
See more at Wikipedia.org...

Noun
1. an agent capable of producing infection
(synonym) infective agent
(hypernym) agent
(hyponym) virus

Any organism, such as a pathogenic virus, parasite, or or bacterium, that is capable of invading body tissues, multiplying, and causing disease.

organisms (such as bacteria, viruses, and fungi) that cause.