Pneumonia is an infection in your lungs and is usually caused by either a bacteria, fungus or a virus. Some types of pneumonia come from inhaling a liquid or chemical. Pneumonia can be serious if its not treated quickly, especially for those who have compromised immune systems, the very young or the very old. Walking pneumonia is the term used for pneumonia that isn't as serious and allows you to "walk around" with it. Because walking pneumonia often feels like a cold or the flu, many people don't even know they have it.