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Infections: At a Glance

By The Manual's Editorial Staff,

Infections are caused by four main types of organisms: bacteria, viruses, fungi, and parasites.

Most infections are caused by microorganisms—tiny living creatures that are present everywhere, including your body. Despite their overwhelming abundance, relatively few of the thousands of species of microorganisms invade, multiply, and cause disease in people.

How Bacteria Shape Up

For a full discussion, see Overview of Infectious Disease.

Antibiotics (antibacterials) are used to treat bacterial infections. They are not effective against viral infections and fungal infections.

Antiviral drugs are used to treat viral infections. Antiviral drugs are much more difficult to develop than antibacterial drugs because viruses have fewer functions that drugs can target.

Several antifungal drugs are available, but the structure and chemical makeup of fungi make fungi difficult to kill.

What Causes Antibiotic Resistance?

Bacteria develop resistance to antibiotics because they acquire genes from other bacteria that have become resistant or because their genes mutate. When antibiotics are used to treat infections, the antibiotics kill the bacteria that lack these genes. Then the remaining resistant bacteria can reproduce and over time outnumber the bacteria that are susceptible to antibiotics.

The more often antibiotics are used, the more likely resistant bacteria are to develop. Therefore, doctors try to use antibiotics only when they are necessary.

Giving antibiotics to people who probably do not have a bacterial infection, such as those who have cough and cold symptoms, does not make people better but does help create resistant bacteria.

Viruses can develop resistance to antiviral drugs.

Did You Know...

Resistant bacteria are common in hospitals because

  • Antibiotics are often needed to treat hospitalized patients.

  • If hospital personnel and visitors do not strictly follow sanitary procedures, they may spread the bacteria.

  • Many hospitalized patients have a weakened immune system, making them more likely to get an infection.

Preventing Infection

Some simple rules can help.

  • Wash your hands before eating and cooking.

  • Store, prepare, and cook foods (particularly meats) appropriately.

  • Cover your mouth and nose with a tissue when you cough or sneeze.

  • Drink only clean or treated water.

  • Practice safe sex.

  • Wash minor wounds with soap and water and keep them covered.

  • Use appropriate clothing and insect repellent when mosquito or tick exposure is likely.

  • Do not use intravenous drugs, and if you do, do not share needles.

See also Cover Your Cough.

Sexually Transmitted Diseases (STDs)

STDs are common in the United States. These infections are typically, but not exclusively, spread through sexual contact. They include chlamydial infections, genital herpes, genital warts, gonorrhea, syphilis, and trichomoniasis. Some infections, such as HIV and hepatitis B, can be transmitted sexually but are commonly transmitted in other ways.

Did You Know...

  • In the United States, about 1.4 million people have genital warts.

STDs often cause sores, warts, or blisters in the area where the organisms entered the body—in the genital area or mouth or around the anus. STDs may cause a discharge from the penis or the vagina, and urination may be painful.

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