What Is Pneumonia?

Pneumonia is a lung infection that can range from mild to so severe that you have to go to the hospital.

It happens when an infection causes the air sacs in your lungs (your doctor will call them alveoli) to fill with fluid or pus. That can make it hard for you to breathe in enough oxygen to reach your bloodstream.

Anyone can get this lung infection. But infants younger than age 2 and people over age 65 are at higher risk. That’s because their immune systems might not be strong enough to fight it.

You can get pneumonia in one or both lungs. You can also have it and not know it. Doctors call this walking pneumonia. Causes include bacteria, viruses, and fungi. If your pneumonia results from bacteria or a virus, you can spread it to someone else.

Lifestyle habits, like smoking cigarettes and drinking too much alcohol, can also raise your chances of getting pneumonia.