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?
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 Resident Flora Healthy people live in harmony with most of the microorganisms that establish themselves on or in (colonize) nonsterile parts of the body, such as the skin, nose, mouth, throat, large intestine... read more .
Other germs don't normally live in or on your body and can make you sick. Examples are the hepatitis virus Acute Hepatitis Hepatitis is inflammation (swelling) of your liver. An acute illness is one that comes on quickly and goes away quickly. Acute hepatitis sometimes becomes chronic hepatitis. A chronic illness... read more or the human immunodeficiency virus (HIV) Human Immunodeficiency Virus (HIV) Infection and AIDS The human immunodeficiency virus (HIV) is a type of virus called a retrovirus. It causes AIDS (acquired immunodeficiency syndrome), which is life-threatening. HIV is called an immunodeficiency... read more .
How do people get infections?
An infection is caused by harmful germs entering your body. They can enter your body through:
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 immune system Overview of the Immune System The immune system is your body's defense system. It helps protect you from illness and infection. The immune system's job is to attack things that don’t belong in your body, including: Germs... read more uses white blood cells White Blood Cells The main components of blood include Plasma Red blood cells White blood cells Platelets read more 
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:
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Diseases that weaken your immune system, such as AIDS, cancer, or diabetes
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Drugs that interfere with your immune system, such as chemotherapy for cancer or corticosteroids
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Medical devices in your body, such as IVs, tubes to drain urine (catheters), breathing tubes in your windpipe, or artificial joints
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Radiation therapy for cancer
What are the symptoms of infection?
Symptoms depend on:
Infection in just one part of your body usually causes pain. 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. 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.
