Giant-cell pneumonia is a deadly, but rare complication of measles. It tends to strike children who are immunodeficient as from leukemia or HIV/AIDS. Note the multinucleated giant cells line the alveoli in the lungs. Viral pneumonia is a pneumonia caused by a virus. Viruses are one of the two major causes of pneumonia, the other being bacteria; less common causes are fungi and parasites. Viruses are the most common cause of pneumonia in children, while in adults bacteria are a more common cause. Magnification: unknown.