The bacterial Streptococcus pneumonia causes most of the cases of bacterial pneumonia in the United States, according to Mayo Clinic. Bacteria-like organisms, virus and fungi also cause pneumonia. In addition, food, drink, saliva or vomit aspirated into the lungs causes aspiration pneumonia.
Viruses such as those that cause the flu or colds also cause viral pneumonia, which is generally mild but may become serious in certain people, states Mayo Clinic. People with weakened immune systems or chronic health conditions often develop pneumonia caused by fungi. The bacteria-like organism Mycoplasma pneumonia causes so-called walking pneumonia, which is pneumonia that is not severe enough to warrant bed rest.