Pneumonia is a lung infection that can make you very sick. You may cough, run a fever, and have a hard time breathing. The infection inflames your lungs’ air sacs, which are called alveoli. The air sacs may fill up with fluid or pus, causing symptoms such as a cough with phlegm (a slimy substance), fever, chills, and trouble breathing. For most people, pneumonia can be treated at home. It...