What Is Pneumonia?

Medically Reviewed

Pneumonia is an infection of the air sacs and surrounding lung tissue. Combined with influenza, it is the eighth leading cause of death in the United States. It is the primary cause of death from infectious disease, claiming the lives of more than 53,000 Americans in 2013 (the most recent year for which statistics are available).

People at increased risk for contracting pneumonia include individuals over age 65, particularly if they have underlying disorders that increase their susceptibility to infection, such as lung cancer or a suppressed immune system (for example, people with HIV or those who take immunosuppressive medications). Smokers, too, are at increased risk for pneumonia.

Based on X-ray results, the various types of pneumonia are often divided into those that affect a single lobe (lobar pneumonia) and those that occur as patches in several lobes (multilobar pneumonia). In addition, pneumonia may involve either one lung (unilateral) or both lungs (bilateral).

What causes 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.

Bacterial pneumonia

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 such as flu, but more often it is a chronic condition such as diabetes, alcoholism, cirrhosis, AIDS, a lung disease such as COPD, or a blood disorder such as leukemia. Also at increased risk are cigarette smokers and individuals who are chronically malnourished or debilitated.

Viral pneumonia

Public health officials estimate that 175,000 cases of pneumococcal pneumonia occur each year in the United States. The fatality rate is 5 to 7 percent, or even higher in older adults.

Approximately half of all pneumonia cases are caused by viruses, not bacteria. Viral pneumonia is most often a complication of the flu, especially in older adults and those with other health problems.

As with 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. Individuals at risk for more severe cases of viral pneumonia 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 a 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


In all types of pneumonia, diagnosis begins with a medical history, a physical examination and a chest X-ray. The responsible organism may be identified by examining a person’s phlegm under a microscope, but treatment should not be delayed while waiting for the analysis.

Other routine laboratory tests for pneumonia include a blood cell count, measurement of electrolyte levels in the blood, urinalysis, liver function tests, and blood cultures. In bacterial pneumonia, the white blood cell count is typically high, though it can fall in severe cases. The white cell count in viral or mycoplasma pneumonia tends to be lower or normal.


While most types of bacterial pneumonia cannot be prevented, vaccines are available that offer some protection against several strains of the bacteria Streptococcus pneumoniae. In addition, flu vaccine has been shown to reduce the risk of viral pneumonia.

The CDC recommends vaccination against both pneumococcal pneumonia and flu for anyone age 65 or older (two types of pneumococcal vaccines are recommended for people in this age group); people with chronic cardiovascular or lung disorders; people with diabetes or chronic liver or kidney disease; smokers; and those with a suppressed immune system due to cancer chemotherapy or conditions such as leukemia or HIV.

Even people who have already had pneumococcal pneumonia should get the pneumococcal vaccine; a Streptococcus pneumoniae infection may confer immunity to one particular strain of the bacterium, but an infection with another strain is still possible. The pneumococcal vaccination schedule varies depending on a number of factors such as your age, health, and type of vaccine you’re receiving. The flu vaccine should be given every autumn.