Infection is a process in which bacteria, viruses, fungi or other organisms enter the body, attach to cells, and multiply. To do this, they must evade 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.
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How Does Infection Occur?
Organisms that can cause illness are all around us: in air, water, soil, and food, as well as in the bodies of animals and other people. Infection occurs when some of them get past a series of natural defenses. Those defenses include:
- Skin: The skin physically blocks germs, but may let them in if it is cut or scraped.
- Coughing deeply: This expels germs from the lungs and breathing passages but may be less effective for weak, sick, or injured people.
- Bacteria: Called "resident flora," harmless bacteria normally are present in some parts of the body. They compete with harmful germs and crowd them out. But they can be weakened or killed by medications, allowing harmful germs to thrive and cause illness.
- Inflammatory response: This is produced by the body's immune system. Certain kinds of white blood cells—including macrophages and neutrophils—surround and destroy or otherwise attack any kind of germs, often causing fever, redness, and swelling.
- Antibodies: These are proteins produced by the immune system. Some are targeted to attack specific microbes. This response is also called humoral immunity. Usually these antibodies are produced after a person is infected by or exposed to the microbe.
The immune system's responses may fail if the germs are too numerous, or if they are too virulent. "Virulent," from the Latin for "poisonous," describes germs that are particularly good at countering the body's defenses. For instance, some microbes can prevent antibodies from forming against them. Another important factor is the functioning 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 microbes.
Where Does Infection Occur?
Localized infections remain in one part of the body. Examples include a cut on the hand that gets infected with bacteria, but does not cause problems anywhere else. Localized infections can be very serious if they are internal, such as in the appendix (appendicitis) or in the heart (endocarditis).
Most serious infections, however, occur when the microorganisms spread throughout the body, usually in the bloodstream. 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, or poisons. Viruses can take over cells and prevent them from doing their normal work. Bacteria and fungi—and larger infective agents like 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 fever, pain, swelling, and fatigue—often is the major cause of the sick feelings an infected person gets.
Do Infections Always Cause Illness?
No, often they do not. Of people infected with tuberculosis bacteria, for instance, only about one in ten will ever get sick. Some viruses and parasites, too, can remain in the body a lifetime without causing illness. In such cases, called latent infection, people usually get sick only if the immune system weakens.
How Do Infections Spread?
The organisms that cause infections may spread through water, soil, food, or air; through contact with an infected person's blood, skin, or mucus; through sexual contact; or through insect bites. Most germs spread by a couple of these routes; no one microbe spreads in all these ways. In addition, many disease-causing microbes can spread from a pregnant woman to her fetus. When this happens, we say the baby is born with a congenital infection.
What Are the Symptoms of Infection?
The symptoms vary greatly depending on the part of the body and type of organism involved. The first sign of bacterial infection is often inflammation: fever, pain, swelling, redness, and pus. By contrast, viral infections less commonly cause inflammation but may cause a variety of other symptoms, from a runny nose or sore throat to a rash or swollen lymph nodes * .
What Is the Treatment for Infection?
The main treatment is usually medication: antibiotics for bacterial infections; antiviral drugs for some viruses (for most there is no treatment); antifungal medications for fungus infections; and antihelmintic drugs for worms. In some cases of localized infection, as when an abscess or collection of pus forms, surgery may be necessary to drain the infected area.
How Are Infections Prevented?
When a wound occurs, infection may be prevented by washing and covering the wound, using antibacterial ointment or spray, and getting medical attention if the wound is serious.
Many systemic infectious diseases can be prevented by immunization. Among them are chickenpox, cholera, diphtheria, hepatitis A and hepatitis B, influenza, Lyme disease, measles, mumps, pertussis (whooping cough), pneumococcal pneumonia, polio, rabies, rubella (German measles), tetanus, typhoid fever, and yellow fever.
Hygiene, sanitation, and public health
Many other systemic infections can be prevented by having a clean public water supply and a sanitary system for disposing of human wastes; by washing hands before handling food; by cooking meats thoroughly; by abstaining from sexual contact; and by controlling or avoiding ticks and mosquitos.
* lymph nodes are round masses of tissue that contain immune cells to filter out harmful microorganisms. During infections, lymph nodes may become enlarged.
U.S. National Institute of Allergy and Infectious Diseases (NIAID),
NIAID Office of Communications and Public Liaison, Building 31, Room
7A-50, 31 Center Drive MSC 2520, Bethesda, MD 20892-2520. NIAID
publishes pamphlets about infectious diseases and posts fact sheets and
newsletters at its website.
The World Health Organization posts fact sheets at its website, covering
communicable/infectious diseases, tropical diseases, vaccine preventable
diseases, and many other health topics.