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Overview of Infectious Disease


The Manual's Editorial Staff

Reviewed/Revised Jan 2024
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What is an infection?

An infection is when microorganisms (germs) invade part of your body and make you sick.

What are microorganisms?

Where do germs come from?

Germs are almost everywhere. There are thousands of different kinds. Some live on your skin or inside your mouth, intestines, and genitals (particularly the vagina). Other germs live on the ground or in water and can enter your body.

Do all germs make people sick?

How do people get infections?

An infection is caused by harmful germs entering your body. They can enter your body through:

  • Your nose or mouth

  • Your skin, from cuts, scratches, or bites

  • Sexual contact with an infected partner

Germs can get in your mouth if you eat or drink something with germs in it. They can also get in your nose or mouth if you touch something contaminated with germs and then touch your nose or mouth. Some germs (typically viruses) are inhaled with respiratory droplets that are produced when a nearby infected person coughs, sneezes, sings, exercises, or talks.

When they get into your body, harmful germs multiply and make you sick.

Sometimes the normal germs in your body get in the wrong place. For example, the normal bacteria in your intestines can cause an infection if they get into your bladder or into your bloodstream.

How does my body defend against infections?

Your body has many ways to defend against infections:

Who's at high risk of infection?

Babies and very old people are more likely to get infections because their body's defenses aren't as strong.

Other things that increase your risk of infection include:

  • Diseases that weaken your immune system, such as AIDS, cancer, or diabetes

  • Medicines that interfere with your immune system, such as corticosteroids or chemotherapy for cancer

  • Medical devices in your body, such as IVs, tubes to drain urine (catheters), breathing tubes in your windpipe, or artificial joints

  • Radiation therapy for cancer

What are the symptoms of infection?

Symptoms depend on:

  • Where the infection is located in your body

  • What type of germ caused your infection

  • Whether just one part or a lot of your body is affected

Infection in just one part of your body usually causes pain and local symptoms. For example, a lung infection (pneumonia Pneumonia Pneumonia is an infection deep in your lungs. The infection involves the small air sacs in your lungs (alveoli). Pneumonia is different from infection of the air passages (bronchi) in your lungs... read more ) can cause chest pain and cough. A brain infection causes headache. An infection under your skin (skin abscess) can swell up, turn red, and be painful.

Infections that affect a lot of your body can cause many different symptoms. Some common general symptoms include:

If you have an infection that isn't treated for a long time, you may lose weight.

How do doctors tell if I have an infection?

Doctors suspect infection based on your symptoms. They usually don't do tests for common infections, such as colds and skin infections. For other infections, doctors often send a sample to the lab to test for germs. Depending on where the infection seems to be, they may send a sample of:

  • Blood

  • Urine

  • Sputum (mucus you cough up)

  • Swabs from your throat, penis, or vagina

How do doctors treat infection?

Your body can fight off some infections on its own.

For other infections, doctors will treat you with medicine to kill the germs. Antibiotics, such as penicillin, are medicines that kill bacteria.

If you have a serious infection, you may need to be in a hospital.

How can I prevent infections?

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