Pneumonia causes inflammation of the air sacs in either one or both lungs. There may be fluid or pus (purulent material) filling the air sacs, causing breathing problems, a fever, chills, and a cough with pus or phlegm. A variety of microorganisms, including bacteria, viruses, and fungi, can cause pneumonia, but a modern virus called COVID-19 has spread over the world and is currently infecting millions of people.