Pneumonia is an infection inside the lungs. It's in the small air sacs (alveoli). It may be caused by a virus, fungus, or bacteria. Pneumonia caused by bacteria is treated with an antibiotic medicine. Severe cases may need to be treated in the hospital. Milder cases can be treated at home. Symptoms may include fever, chills, and cough (dry or with phlegm). You may have a headache, muscle weakness, trouble breathing, and pain. These symptoms often get worse in the first 2 days. But they often start to get better in the first week of treatment.