In general biology, a germ refers to the small mass of cells from which a new organism or part of an organism develops; a seed, bud, spore, or zygote.

Perhaps its more common use is when referring to a pathogen: a pathogenic microorganism, such as a bacterium or virus. "Germ" in this sense came from the idea that these miniscule bodies are the origin of some disease, which is known as the "the germ theory of disease."