germ, microbe, bacteria, virus

These terms are so closely related that only a scientist would ordinarily need to differentiate among them. A germ is a microbe, a disease-producing microorganism. In biology, germ refers to an initial stage in development, as "a germ cell of such-and-such a form." By extension, germ also is used to refer to anything that acts as a source or initial stage: "I have the germ of an idea." A microbe is a bacterium, the singular form of bacteria, which refers to any of numerous microscopic organisms involved in such processes as fermentation, putrefaction, and disease. A virus is an infectious agent, especially one that reproduces only in living cells. Virus is really a medical term for the active element that infects with and produces a contagious disease: "the virus of yellow fever." Germ is a general term which will serve in all ordinary situations.