Pneumonia has many possible causes. The most common are bacteria and viruses in the air we breathe. Your body usually prevents these germs from infecting your lungs. But sometimes these germs can overpower your immune system, even if your health is generally good.

Pneumonia is classified according to the types of germs that cause it, and where you acquired the infection.

Community-acquired pneumonia

Community-acquired pneumonia is the most common type of pneumonia. It occurs outside of hospitals and other health care facilities, and may be caused by:

  • Bacteria, such as Streptococcus pneumoniae. Bacterial community-acquired pneumonia can occur on its own or after you have a cold or respiratory flu. This type of pneumonia often affects one area (lobe) of the lung, a condition called lobar pneumonia.
  • Bacteria-like organisms, such as Mycoplasma pneumoniae, which typically produce milder signs and symptoms than do other types of pneumonia. "Walking pneumonia," a term used to describe pneumonia that isn't severe enough to require bed rest, may result from Mycoplasma pneumoniae.
  • Viruses, including some that are the same type of viruses that cause colds and flu. Viruses are the most common cause of pneumonia in children younger than 2 years. Viral pneumonia is usually mild. But viral pneumonia caused by certain influenza viruses, such as sudden acute respiratory syndrome (SARS), can become very serious.
  • Fungi, which can be found in soil and in bird droppings. This type of pneumonia is most common in people with an underlying health problem or weakened immune system and in people who have inhaled a large dose of the organisms.

Hospital-acquired pneumonia

Hospital-acquired pneumonia is a bacterial infection that occurs in people 48 hours or more after being hospitalized for another condition. Hospital-acquired pneumonia can be serious because the bacteria causing it may be more resistant to antibiotics.

People who are on a breathing machine (ventilator), often used in intensive care units, are at higher risk of this type of pneumonia.

Health care-acquired pneumonia

Health care-acquired pneumonia is a bacterial infection that occurs in people who are living in long-term care facilities or have been treated in outpatient clinics, including kidney dialysis centers. Like hospital-acquired pneumonia, health care-acquired pneumonia can be caused by bacteria more resistant to antibiotics.

Aspiration pneumonia

Aspiration pneumonia occurs when you inhale food, drink, vomit or saliva into your lungs. This aspiration may happen if something disturbs your normal gag reflex, such as a brain injury, swallowing problem, or excessive use of alcohol or drugs.

May. 21, 2013

You Are ... The Campaign for Mayo Clinic

Mayo Clinic is a not-for-profit organization. Make a difference today.