In biology, a<unk>  (, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism that can produce disease. A<unk>  may also be referred to as an infectious agent, or simply a germ. The term<unk>  came into use in the 1880s. Typically, the term is used to describe an infectious microorganism or agent, such as a virus, bacterium, protozoan, prion, viroid, or fungus. Small animals, such as certain worms or insects, can also cause or transmit disease. However, these animals are usually, in common parlance, referred to as parasites rather than<unk> s. The scientific study of microscopic organisms, including microscopic<unk> ic organisms, is called microbiology, while parasitology refers to the scientific study of parasites and the organisms that host them. There are several pathways through which<unk> s can invade a host. The principal pathways have different episodic time frames, but soil has the longest or most persistent potential for harboring a<unk> . Diseases in humans that are caused by infectious agents are known as<unk> ic diseases. Not all diseases are caused by<unk> s, other causes are, for example, toxins, genetic disorders and the host's own immune system.
Pathogen
Disease vector
Pollutant
Biological agent
Microorganism
Toxicant
Bioprocess
Chemical substance