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Overview of Infectious Disease

By Allan R. Tunkel, MD, PhD

Microorganisms are tiny living creatures, such as bacteria and viruses. Microorganisms are present everywhere. Despite their overwhelming abundance, relatively few of the thousands of species of microorganisms invade, multiply, and cause disease in people.

Many microorganisms live on the skin and in the mouth, upper airways, intestine, and genitals (particularly the vagina) without causing disease. Whether a microorganism lives as a harmless companion to a person or invades and causes disease depends on the nature of the microorganism and on the state of the person’s natural defenses.

Types of Infectious Organisms




Some Disorders That Can Result


Bacteria are microscopic, single-celled organisms without a nucleus.

Streptococcus pyogenes

Pharyngitis (strep throat)

Escherichia coli

Urinary tract infections


Viruses are small infectious organisms—much smaller than a fungus or bacterium. They cannot reproduce on their own. They must invade a living cell and use that cell’s machinery to reproduce.

Varicella zoster

Chickenpox and shingles


The common cold


Fungi are neither plants nor animals. Their size ranges from microscopic to easily seen with the naked eye. They include yeasts, molds, and mushrooms.

Candida albicans

Vaginal yeast infections

Tinea pedis

Athlete’s foot


Parasites are organisms that survive by living inside another, usually much larger organism (the host). They include worms and single-celled organisms called protozoa (which, unlike bacteria, have a nucleus) .

Enterobius vermicularis (a species of pinworm)

Itching around the anus

Plasmodium falciparum (a species of protozoa)


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