Hillary Clinton, the Democratic presidential nominee, has pneumonia, her doctor said in a statement on Sunday.

Here are some facts about the illness: what it is, what it does and how people recover.

What is pneumonia?

Pneumonia is a common lung infection that affects millions of people each year. It is most often caused by bacteria, though viruses account for up to one-third of the cases in the United States each year, according to the National Heart, Lung and Blood Institute. Fungi and other germs can also cause pneumonia.

When germs reach the lungs, the immune system responds by sending cells to attack them. Those cells can cause air sacs in the lungs, known as alveoli, to become inflamed or fill with fluid and pus, causing the symptoms of pneumonia.