Share this page:   Pneumonia can be due to a variety of bacteria
and viruses, less commonly due to fungi, and rarely due to other
microorganisms or parasites. Likely causes are associated with
the age of the person, the season, the person's health status,
and/or where the infection was contracted.  Some of the more
common examples of microorganisms that cause pneumonia are
listed in the table below:  Type Comments Some Causative
Organisms Viral Viruses cause about one-third of all cases of
pneumonia in the US each year.  They are the most common cause
of pneumonia in infants, accounting for approximately 90% of all
lower respiratory infections.  Less than 20% of pneumonias in
adults younger than 60 years old are viral.  In those over 60-65
years of age, viral causes become increasingly common.  Viral
pneumonia is often mild, requiring only supportive care.
However, more serious cases may require hospitalization.
Sometimes bacterial pneumonia may develop following viral
pneumonia.  The most common causes of vi