Introduction: Biology of Infectious Disease: Merck Manual Home Edition


	Section		Infections

	Subject		Biology of Infectious Disease

Introduction

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 produce illness 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

Bacteria: Bacteria are microscopic, single-celled organisms. Examples: Streptococcus pyogenes (strep throat); Escherichia coli (urinary tract infection).

Viruses: A virus is a small infectious organism—much smaller than a fungus or bacterium—that cannot reproduce on its own; it must invade a living cell and use that cell's machinery to reproduce. Examples: Varicella zoster (chickenpox, shingles); Rhinovirus (common cold).

Fungi: Fungi are actually a type of plant. Yeasts, molds, and mushrooms are all types of fungi. Examples: Candida albicans (vaginal yeast infection); Tinea pedis (athlete's foot).

Parasites: A parasite is an organism, such as a worm or single-celled animal (protozoan) that survives by living inside another, usually much larger, organism (the host). Examples: Enterobius vermicularis (pinworm); Plasmodium falciparum (malaria).

Last full review/revision February 2003