This article is from the Asthma FAQ, by Patricia Wrean and Marie Goldenberg firstname.lastname@example.org with numerous contributions by others.
Pneumonia is an infection of the lung tissue. In adults, it is
generally caused by bacterial infections, though viruses, fungi,
and protozoa may also be culprits. The latter microorganisms
have become very common as causes of pneumonia in immunosuppressed
persons, such as those with HIV infection. However, for those
with chronic illnesses, especially cardiac or respiratory
diseases, or those at increased risk for pneumonia, there is a
pneumococcal pneumonia vaccination available as a preventive
measure for the most common of these bacterial infections,
streptococcus pneumoniae. In children, pneumonia is most
commonly caused by viruses.