Pneumonia is a sickness that affects your lungs. It happens when germs, like bacteria or viruses, get into your airways and lung sacs. These germs can make you very sick and make it hard for your lungs to work. Your lungs can fill with fluid or inflammation, and then it's hard for your body to take the oxygen it needs to feel better. To get better, you usually have to take medicine or go to the hospital for help.