What Can Cause Pneumonia?

Hundreds of different microorganisms can infect the lungs. Viruses and bacteria are the most common infectious agents that cause acute pneumonia, but other organisms (for example, Mycoplasma pneumoniae, Mycobacterium tuberculosis, fungi, and the parasite Pneumocystis carinii) can be responsible, especially if a person's immune defenses are compromised.

Environmental exposure and the setting in which pneumonia occurs (for example, in the community, a nursing home, or a hospital) are major determinants of the type of pneumonia and the type of microorganism responsible for the disease.

The most common type of bacterial pneumonia is pneumococcal pneumonia, which is caused by the bacterium Streptococcus pneumoniae. Pneumococcal pneumonia can spread from person to person (by coughing, for example), especially when people live in close quarters, as is the case in nursing homes, military barracks, and prisons.

Streptococcus pneumoniae exists harmlessly in the throats of many healthy people. Most people who develop pneumococcal pneumonia have some underlying disease that renders them suddenly susceptible to the infection. This may be an acute infection like flu, but more often it is a chronic condition such as

Also at increased risk are cigarette smokers and individuals who are chronically malnourished or debilitated. Pneumococcal pneumonia is far more common in the elderly and more dangerous: It kills approximately 5,000 people in the United States each year, and nearly half are older adults.

Half of all pneumonia cases are caused by viruses, not bacteria. Most often, viral pneumonia is a complication of flu, especially in older adults and those with other health problems. Like bacterial pneumonia, a virus that causes pneumonia can spread through close contact with an infected person.

Most cases of viral pneumonia are mild and will get better on their own. People at risk for more severe cases include the elderly, young children, and those with impaired immune systems (such as transplant recipients or people with HIV). People with viral pneumonia may also develop a bacterial infection, which can worsen their illness.

Pneumonia Symptoms

In bacterial pneumonia, symptoms typically begin abruptly. They may include

  • cough that produces yellow phlegm
  • high fever
  • chills
  • sharp chest pain brought on by breathing or coughing, and
  • shortness of breath.

Young people usually have increased breathing and heart rates and appear acutely ill. Older adults, however, typically have fewer symptoms, experiencing lethargy and confusion but often no fever and, sometimes, no lung-related symptoms.

The early symptoms of viral pneumonia may resemble those of influenza, including a dry cough, fever, headache, muscle pain, and weakness. Twelve to 36 hours later, however, people with viral pneumonia may experience increased breathlessness, worsening cough with mucus, high fever, and, possibly, blueness of the lips. People who have very serious viral pneumonia may experience extreme difficulty breathing.

Publication Review By: Peter B. Terry, M.D., M.A.

Published: 19 Sep 2011

Last Modified: 12 Feb 2015