Find information on medical topics, symptoms, drugs, procedures, news and more, written in everyday language.

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

Type

Description

Examples

Some Disorders That Can Result

Bacteria

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

Streptococcus pyogenes

Pharyngitis (strep throat)

Escherichia coli

Urinary tract infections

Viruses

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

Rhinovirus

The common cold

Fungi

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

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)

Malaria

Resources In This Article