What three things can cause the lung infection pneumonia?

Bacteria, viruses, Bacteria, viruses and fungi cause pneumonia. That is to say, there are hundreds of thousands of causes of pneumonia if not more.
Diff germs. Pneumonia ( lung infection) occurs when some type of microbe( that is "germ") gets into the lungs. Germs include bacteria, viruses or fungi. Germs can be "aspirated" (get into the lungs when oral secretions with the bacteria get into the airway) or airborne germs ( fungus or virus) get inhaled. Some underlying lung problems weaken defenses and increase the risk for an infection. (copd, aids etc.