Germ theory, in medicine, is the scientific principle that infectious diseases are caused by microorganisms—such as bacteria, viruses, fungi, and protozoa—that invade and multiply within the body. These organisms are too small to be seen without a microscope and can spread between individuals through various forms of contact or environmental exposure. Germ theory established that specific pathogens cause specific diseases, forming the basis for modern practices such as sanitation, vaccination, and infection control.
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