germ

Germ typically refers to a microorganism: bacteria, viruses, fungi, or protozoa. They are found in air, water, soil, foods, and on surfaces. Most microorganisms are invisible to the naked eye and many are harmless or beneficial, but some can cause illness, infections, or food spoilage. The germ theory of disease posits that many diseases are caused by microorganisms that invade the body, a idea that helped establish practices such as sanitation, vaccination, and antibiotics. Public health measures aim to reduce the transmission of germs through hygiene, disinfection, sterilization, and safe food handling.

Germ also has botanical meaning: in a seed, the germ is the embryo—the part that develops into a new plant. The process of growth from a seed to a seedling is called germination, during which the embryo resumes growth and emerges from the seed coat. The term is sometimes used more broadly to refer to the origin or source of something, as in "the germ of an idea."

In genetics and developmental biology, the term germ is used in phrases such as germ cells, which give rise to gametes, and germ line, the lineage of cells that passes genetic information to offspring. These uses reflect the sense of origin or birth of biological material.