Germ, general term employed loosely to designate any minute pathogenic agent. The term is applied to disease-producing microorganisms, such as bacteria, protozoa, and fungi, and to pathogenic agents of uncertain classification, such as rickettsia and viruses (see Virus).

The term germ became widely used after the development of the germ theory of disease in the 19th century. Scientists and science writers after the beginning of the 20th century have tended to use the specific technical names of particular microorganisms.