Pneumonia (say: new-mo-nya) is an infection of one or both lungs. To know what that means, you have to know something about lungs and what they do. When you breathe in, you pull oxygen into your lungs. That oxygen travels through breathing tubes and eventually gets into your blood through the alveoli (say: al-vee-oh-lie).
Alveoli are tiny air sacs covered in tiny blood vessels called capillaries. How can something so small get oxygen into your blood? You have about 600 million of them! When oxygen-rich air reaches the alveoli, it can be absorbed into the blood. Then your red blood cells can carry oxygen all over your body. The body needs oxygen to keep working properly and to stay alive.
But if a person has pneumonia, his or her lungs can't do this important job very well. Why? Because this kind of infection creates fluid that blocks the alveoli. This makes it hard for oxygen to get deep into the lungs, where it can be passed through to the blood. The person can still breathe, but it might be hard to breathe, especially if the pneumonia affects both lungs.
Pneumonia can happen to people at any age, from tiny babies to really old people. Getting wet doesn't cause pneumonia, but an infection from bacteria or a virus does. A cold or flu that gets worse can turn into pneumonia. That's because the cold or flu will irritate the lungs, creating an environment where it's easier for pneumonia germs to move in and start an infection.