Germs are the oldest living things on Earth, literally billions of years older than mammals, including the most highly advanced mammal-humans. Therefore, early humans, other mammals, and their more primitive ancestors all lived out their lives with germs on them, inside them, and inhabiting every conceivable niche of the environment around them. Some of these germs were harmless and simply coexisted with plants and animals. Others proved more dangerous by infecting and killing other living things. Indeed, diseases caused by germs are nothing new, as noted science writer Arno Karlen points out in his popular book Man and Microbes:

Infection was already ubiquitous [existing everywhere] when higher organisms left their first fossil traces, some half a billion years ago. There are fossil plants with fossil fungus infections, and ancient jellyfish and mollusks bearing signs of parasites. Dinosaur bones 250 million years...