Final answer:
The germ theory of disease explains how microorganisms cause contagious diseases by invading human hosts and making them sick.
Explanation:
The germ theory of disease states that contagious diseases are caused by "germs," or microorganisms, which are organisms that are too small to be seen without magnification. Microorganisms that cause disease are called pathogens. When pathogens invade humans or other living hosts, they grow, reproduce, and make their hosts sick. Diseases caused by germs are contagious because the microorganisms that cause them can spread from person to person.
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