I am not a doctor but I have Googled your question and this is what I found:
"Pneumonia in children and adults is caused by bacteria, virus, fungi or protozoa. Out of these, bacteria are the most common microorganisms, that have been found to cause pneumonia. These microorganisms usually enter the body through mouth, nose and eyes, and then infect the lungs, especially the air sacs or alveoli of the lungs. Pneumonia is more commonly found to affect the lower lobes of the lungs.