Pneumonia - Causes

Atypical Pneumonia

Atypical pneumonias produce mild symptoms and a dry cough. Organisms that cause atypical pneumonias include:

Mycoplasma pneumoniae (M. pneumoniae) is the most common atypical pneumonia organism. Mycoplasma is a very small bacterium that lacks a cell wall. Pneumonia caused by M. penumoniae spreads when someone carrying the infection is in close contact with others for a long period of time. It is most often found in school-aged children and young adults. The condition, commonly called "walking pneumonia," is usually mild.
Chlamydia pneumoniae (C. pneumoniae) is now thought to cause 10% of all community-acquired cases of pneumonia. This atypical pneumonia is most common in young adults and children,and it is usually mild.It isless common, but usually more severe, in the elderly.
Legionella pneumophilacauses Legionnaires' disease. You can catch it bybreathing in drops ofcontaminated water. Outbreaks have most often been reported in hotels, cruise ships, and office buildings where people are exposed to contaminated droplets from cooling towers and evaporative condensers. They have also been reported in people who have been near whirlpools and saunas. Legionella pneumophilais not passed on from person to person. Some experts believe the organismcauses 29% to 47% of all pneumonia cases.

Legionnaires' disease organism, legionella

Legionnaires' disease was first described in 1976 after and outbreak of fatal pneumonia at an American Legion convention. The newly described organismthat caused the disease was named Legionella pneumophila, shown in this picture. (Courtesy of the Centers for Disease Control.)

Viral Pneumonia

A number of viruses can cause pneumonia either directly or indirectly. They include:

Influenza (Flu). Pneumonia isa major serious complication of theflu and can be very serious. It can develop about five days after flu symptoms start. The flu weakens the body's defense systems, making it easier for bacteria to grow in the lungs.
Respiratory syncytial virus (RSV). Most infants are infected with RSV at some point, but it is most often mild. Still, RSV is a major cause of pneumonia in infants as well as people with damaged immune systems.Studies indicate that RSV pneumonia maybe more common in adults than previously thought, especially the elderly.
Severe acute respiratory syndrome (SARS). SARS is a respiratory infection caused by a newly-described coronavirus, which appears to have jumped from animals to humans.The disease was firstreported in China in 2003.
Human parainfluenza virus. This virus is a leading cause of pneumonia and bronchitis in children, the elderly and in patients with damaged immune systems.
Adenoviruses. Adenoviruses are common and usually are not problematic, although they have been linked to about 10% of childhood pneumonia.
Herpesviruses. In adults, herpes simplex virus and varicella-zoster (the cause of chicken pox) can cause pneumonia in people with impaired immune systems.