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Quick Facts

Overview of Infectious Disease


The Manual's Editorial Staff

Last full review/revision Feb 2020| Content last modified Feb 2020
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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?

"Micro" means small, so microorganisms are tiny living creatures. They're so small they can't be seen without a microscope. There are many different kinds of microorganisms, including:

The term "germs" refers to the different types of microorganisms that can cause disease.

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?

Many germs don't cause infection. Some are even helpful. Many of the germs that live on your skin or in your body are normal and don't hurt you. These are called your resident flora.

Other germs don't normally live in or on your body and can make you sick. Examples are the hepatitis virus or the human immunodeficiency virus (HIV).

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.

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:

  • Your skin keeps germs out

  • The mucus in your nose, throat, eyes, vagina, and intestines washes germs away and contains substances that can kill them

  • Your immune system attacks germs inside your body

  • Fever (high body temperature) helps kill germs

Your immune system uses white blood cells to recognize harmful germs. Some white blood cells kill germs directly. Others make substances called antibodies that kill germs.

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

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

  • 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:

  • 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. For example, a lung infection (pneumonia) can cause chest pain. 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:

  • Feeling weak and tired

  • Poor appetite

  • Aching all over

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?

To prevent infections:

  • Wash your hands often, especially if you handle food or have a lot of contact with people

  • Get the vaccinations your doctor says you need

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