Infection is a process in which bacteria * , viruses * , fungi * , or other microorganisms * enter the body, adhere to or enter cells, and multiply. To do this, they must get around or overcome the body's natural defenses at each step. Infections have the potential to cause illness, but in many cases, the infected person does not get sick.

How Does Infection Occur?

Microorganisms that can cause illness are everywhere in the environment: in air, water, soil, and food, as well as in the bodies of animals and other people. Infection occurs when some of the invading microorganisms get past a series of natural defenses. Those defenses include the following:

The immune system's responses may fail if the microorganisms are too numerous or if they are too virulent. Virulent, from the Latin for “poisonous,” describes microorganisms that are particularly good at countering the body's defenses. For instance, some microorganisms can prevent formation of those antibodies that might have targeted them. Another important factor is the level of competence of the immune system. If it is damaged (weakened, for instance, by age or illness), infection is more likely. Babies tend to get more infections because their immune systems have not yet learned to recognize and attack some microorganisms.

Where Does Infection Occur?

Localized infections

A localized infection remains in one part of the body. An example is a cut on the hand that leads to a bacterial infection but does not cause problems elsewhere in the body. Localized infections can be very serious if they are internal, for example, appendicitis (localized to the appendix) and endocarditis (localized to the interior wall of the heart).

Systemic infections

Most serious infections, however, occur when the microorganisms travel throughout the body, usually by way of the bloodstream, and the sites of infection in the body are multiple. These are called systemic infections, and they include flu, malaria * , AIDS * , tuberculosis * , plague * , and most of the infectious diseases whose names are familiar.

How Do Infections Lead to Illness?

The major causes of infection are viruses, bacteria, fungi, and parasites, including protozoa (one-celled organisms), worms, and insects such as mites (which cause scabies) and lice.

Bacteria can release toxins (poisons). Viruses can take over cells and prevent them from doing their normal work. Bacteria and fungi—and larger infective agents such as worms or other parasites—can multiply so rapidly that they physically interfere with the functioning of the lungs, heart, or other organs. The immune response itself—which can bring on fever, pain, swelling, and fatigue (as by-products of the effort to overcome the offending agent)—often is the major cause of the sick feelings an infected person gets.

Do Infections Always Cause Illness?

How Do Infections Spread?

The microorganisms that cause infections are present in water, soil, food, and air; they may be transmitted through contact with these substances; through contact with an infected person's blood, skin, mucus, or respiratory secretions expelled during coughing or sneezing; through sexual contact; or through insect bites. Many microorganisms are spread by two or more of these routes. No one microorganism spreads in all these ways. In addition, many disease-causing microorganisms can be transmitted, in pregnant women, from mother to fetus, called a congenital infection in the newborn infant.

What Are the Symptoms of Infection?

The symptoms vary greatly depending on the part of the body and type of microorganism involved. The first sign of bacterial infection is often inflammation: fever, pain, swelling, redness, and partial loss of function. By contrast, viral infections less commonly cause inflammation but may cause a variety of other symptoms, ranging from a runny nose or sore throat to a rash or swollen lymph nodes * .

What Is the Treatment for Infection?

The specific treatment depends on the microorganism that is involved. Doctors prescribe antibiotics for bacterial infections, antiviral drugs for some viral infections (for most, however, no treatment exists), antifungal * medications for fungus infections, and anthelmintic drugs for worms. The doctor's decision to use (or not use) medications may also depend on the exact location of the infection, as well as the age and the general medical condition of the patient. In some cases of localized infection, as when an abscess or collection of pus * forms, the patient may need surgery to drain the infected area.

How Are Infections Prevented?

Disinfecting wounds

When a wound occurs, prompt and thorough cleansing of the fresh wound by placing it under running water is one of the most effective ways to prevent infection. Additional methods of preventing infection include using antibacterial ointment or spray and covering the wound. Serious wounds should receive immediate medical attention.

Immunization Hygiene, sanitation, and public health

Individuals can prevent many other systemic infections by washing hands before handling food; cooking meats thoroughly; using safe sex techniques; and controlling or avoiding ticks and mosquitoes. In addition, a clean public water supply and a system for the sanitary disposal of human wastes can help to prevent infections. While clean water supplies and sewage systems are typical in developed countries, many developing countries have neither. To prevent or reduce the incidence of sexually transmitted infection, one should be responsible, practice safe sex, and know their partner.

See also Bacterial Infections • Fungal Infections • Parasitic Diseases: Overview • Roundworm Infection • Viral Infections


Books and Articles

Gangat, M.A., and J.L. Hsu. “Antibiotic Stewardship: A Focus on Ambulatory Care.” South Dakota Medicine 44, no. 8 (2015). (accessed July 20, 2015).

McKenna, Maryn. “Antibiotic Resistance: The Last Resort.” Nature. July 24, 2013. (accessed July 20, 2015).

U.S. Food and Drug Administration. Bad Bug Book: Foodborne Pathogenic Microorganisms and Natural Toxins Handbook, 2nd ed. Silver Spring, MD: FDA, 2012. (accessed July 20, 2015).


Centers for Disease Control and Prevention. “Fungal Diseases.” CDC. gov. (accessed July 20, 2015).

MedlinePlus. “Viral Infections.” U.S. National Library of Medicine. (accessed July 20, 2015).


Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30333. Toll-free: 800-CDC-INFO. Website: (accessed July 20, 2015).

National Institute of Allergy and Infectious Diseases. Office of Communications and Public Liaison, 6610 Rockledge Dr., MSC 6612, Bethesda, MD 20892-6612. Toll-free: 866-284-4107. Website: (accessed July 20, 2015).

World Health Organization. Avenue Appia 20, CH 1211 Geneva 27, Switzerland. Telephone: 41 22 791 2111. Website: (accessed July 20, 2015).

* bacteria (bak-TEER-ee-a) are single-celled microorganisms, which typically reproduce by cell division. Some, but not all, types of bacteria can cause disease in humans. Many types can live in the body without causing harm.

* viruses (VY-rus-sez) are tiny infectious agents that can cause infectious diseases. Viruses can only reproduce within the cells they infect.

* fungi (FUNG-eye) are microorganisms that can grow in or on the body, causing infections of internal organs or of the skin, hair, and nails.

* microorganisms are tiny organisms that can only be seen using a microscope.

* malaria (mah-LAIR-e-uh) is a disease spread to humans by the bite of an infected mosquito.

* AIDS, or acquired immunodeficiency (ih-myoo-no-dih-FIH-shen-see) syndrome, is an infection that severely weakens the immune system; it is caused by the human immunodeficiency virus (HIV).

* tuberculosis (too-ber-kyoo-LO-sis) is a bacterial infection that primarily attacks the lungs but can spread to other parts of the body.

* plague (PLAYG) is a serious bacterial infection that is spread to humans by infected rodents and their fleas.

* lymph nodes (LIMF) are small, bean-shaped masses of tissue containing immune system cells that fight harmful microorganisms. Lymph nodes may swell during infections.

* antifungal drugs (an-ty-FUNGal) are medications that kill fungi.

* pus is a thick, creamy fluid, usually yellow or ivory in color, that forms at the site of an infection. Pus contains infection-fighting white cells and other substances.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)