How Infectious Diseases Work

The human body is both surrounded and inhabited by billions of microorganisms (living organisms that are so small that they can be seen only with a microscope). Most microorganisms are harmless or even beneficial; for example, bacteria that normally live in the digestive system help digest food. Occasionally, however, a microorganism capable of causing a disease invades the body. Diseases caused by such microorganisms are called infectious diseases.

How Infectious Diseases Work
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