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Germs live everywhere. There are germs, also called microbes, in the air; on food, plants and animals; and in the soil and water. Germs are on just about every surface, including the human body.

Most germs cause no harm. The immune system protects against things that cause infections, called infectious agents. But some germs keep changing to get past the immune system's defenses. Knowing how germs work can increase your chances of not getting infections.