The National Academies

The National Academies: What You Need To Know About Infectious Disease

What You Need To Know About Infectious Disease

How Infection Works

Types of Microbes

The microorganisms, or microbes, that can cause disease come in different forms. Viruses and bacteria are probably the most familiar because we hear so much about them. But fungi, protozoa, and helminths are also big players in the story of infectious disease. Learn more about each of these five main categories, as well as a recently discovered one: prions.



Viruses are unable to reproduce until they invade and commandeer living cells.

Influenza, measles, and the common cold are just some of the diseases caused by viruses. What is a virus and how is it different from other microbes?

More about viruses



Bacteria come in three shapes: spherical, rodlike, and curved.

Bacteria are single-celled organisms that have been around for billions of years. Discover their important characteristics.

More about bacteria

Other Microbes

Other Microbes

Bread mold and hookworm, both infectious agents, are neither bacteria nor viruses.

Viruses and bacteria may be the most recognizable of the microbes that can cause infectious disease. But there are several other varieties. Learn about them here.

More about other microbes

Explore Other Topics

What do you know about infectious disease?

Which are larger?

  • Correct!

    Bacteria are 10 to 100 times larger than viruses.

  • Sorry, that’s incorrect.

    Bacteria are 10 to 100 times larger than viruses.

Infectious Disease Defined


An organism (usually an arthropod such as a flea, mosquito, or tick) that carries an infectious agent from one host to another.

View our full glossary

National Academies

Search the National Academies Press website by selecting one of these related terms.