Pneumonia is a serious disease that involves inflammation of the lungs and lower airways. It usually also affects the upper respiratory tract - the nasal cavities, throat, trachea (“windpipe”) and bronchi. The lungs supply blood with oxygen from inhaled air and expel carbon dioxide in exhaled air. Pneumonia can be caused by bacteria, viruses, fungi, internal parasites and other noxious substances that get into the lungs when a dog breathes them in. Fortunately, pneumonia isn’t common in healthy adult dogs. It mainly affects the very young, the very old and dogs with compromised immune systems. Some dogs with pneumonia never show signs of being sick. Other dogs develop symptoms because insufficient oxygen is being circulated in their bloodstream. They will breathe rapidly, noisily and with some difficulty. They often have a wet cough and a thick nasal discharge from both nostrils. They may sneeze and run a fever, lose their appetite and become weak, depressed and lethargic.