Pneumonia - Causes

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Atypical Pneumonia

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

• Mycoplasma (M.) pneumoniae, the most common atypical pneumonia organism. Mycoplasma is a very small bacterium that lacks a cell wall. Pneumonia caused by M. pneumoniae spreads when someone carrying the infection comes 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 (C.) pneumoniae is now thought to cause 10% of all CAP cases. This atypical pneumonia is most common in young adults and children, and is usually mild. It is less common, but usually more severe, in the elderly.
• Legionella pneumophila causes Legionnaires' disease. It is contracted by breathing in drops of contaminated water. Outbreaks are often 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 pneumophila is not passed from person to person. Some experts believe the organism causes 29 - 47% of all pneumonia cases.

Viral Pneumonia

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

• Influenza (Flu). Pneumonia is a major complication of the flu (including H1N1, or swine flu) and can be very serious. Influenza-associated pneumonia is particularly risky for the elderly and people with heart disease. It can develop about 5 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. However, RSV is a major cause of pneumonia in infants, as well as in adults with damaged immune systems. Studies indicate that RSV pneumonia may be more common in adults, especially the elderly, than previously thought.
• Severe acute respiratory syndrome (SARS). SARS is a respiratory infection caused by a coronavirus, which appears to have jumped from animals to humans. The disease was first reported in China in 2003 but has not been a problem in recent years.
• Human parainfluenza virus. This virus is a leading cause of pneumonia and bronchitis in children, the elderly, and patients with damaged immune systems.
• Adenoviruses. Adenoviruses are common and usually are not problematic, although they have been linked to about 10% of childhood pneumonias. Adenovirus 14 has been linked to an outbreak of severe community-acquired pneumonia in the Pacific northwest.
• Herpes viruses. In adults, herpes simplex virus and varicella zoster (the cause of chickenpox) can cause pneumonia in people with impaired immune systems.
• Avian influenza. Type A influenza subtype H5N1 in birds is spreading around the globe. Fortunately, only a few hundred human cases have been identified. Most have resulted from close contact with infected birds. The virus does not seem to spread easily from person to person. All patients diagnosed with "bird flu" show signs of pneumonia, although symptoms may be mild. Oseltamivir (Tamiflu) is the most effective treatment for this type of influenza, which can be fatal.