Merck Manual

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Overview of Infectious Disease


Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Last full review/revision Jul 2020| Content last modified Jul 2020
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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, intestines, and genitals (particularly the vagina) without causing disease (see Resident Flora). 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 (see Defenses Against Infection and see Defenses Against Infection).


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

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


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


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)

Plasmodium falciparum (a species of protozoa)

Certain microorganisms have the potential to be used as biological weapons. These microorganisms include those that cause anthrax, brucellosis, hemorrhagic fever (Ebola virus infection and Marburg virus infection), plague, smallpox, and tularemia and those that produce botulinum toxin. Each of the diseases is potentially fatal and, except for anthrax, botulism, and tularemia, can be passed from person to person. Direct person-to person transmission of brucellosis is extremely rare.

Effects of Infection

Infections may affect only part of the body (a local infection) or the whole body (a systemic infection). Abscesses and urinary bladder infections are examples of local infections. Severe systemic infections may have life-threatening effects, such as sepsis or septic shock.

Symptoms of infection can include fever, a racing pulse, faster breathing, anxiety, and confusion.

Most effects resolve when the infection is effectively treated.

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