Causes of Pneumonia

In up to 30 percent of adult cases, community-acquired pneumonia (the most common type) is caused by a virus. This percentage is much higher for children (viruses are thought to be the most common cause in this age groups). Viruses are not the only germs associated with the disease, however. Bacteria, fungi, or even parasites have also been known to cause pneumonia.

What Causes Pneumonia?

Pneumonia is an infection in the lungs caused by certain types of bacteria, viruses, or (less commonly) fungi. It is often separated into different types based on where the infection occurred and what caused it.
This article will look at the different causes for both community-acquired pneumonia and hospital-acquired pneumonia.

Causes of Community-Acquired Pneumonia

Community-acquired pneumonia (CAP) is the most common type of pneumonia. It affects almost four million people each year. About one out of every five people who has CAP needs to be treated in a hospital.
There are more than 100 types of germs (bacteria, viruses, fungi, and parasites) known to cause CAP. In everyday practice, the germ causing the illness is actually determined only about 20 percent of the time. Most of the time, pneumonia is treated based on the germs most likely to be causing the infection.
Bacterial Community-Acquired Pneumonia
The bacteria most likely to result in CAP include:
  • Streptococcus pneumoniae, also known as just pneumococcus (the most common cause of pneumonia worldwide)
  • Mycoplasma pneumoniae
  • Haemophilus influenzae
  • Chlamydophila pneumoniae
  • Legionella pneumophila.
Mycoplasma pneumonia is a common type of pneumonia that usually affects people younger than 40. It is a common cause of walking pneumonia.
People who live or work in crowded places like schools, homeless shelters, and prisons are most likely to get this type of pneumonia. It's usually mild and responds well to treatment with antibiotics. However, it can be quite serious in some people. The disease may be associated with a skin rash and hemolysis (the breakdown of red blood cells).
Chlamydophila pneumoniae is a kind of pneumonia that can occur all year. The infection, which is generally mild, is most common in people 65 to 79 years of age.
Legionella pneumophila is sometimes called Legionnaire's disease. This type of pneumonia has caused serious outbreaks that have been linked to exposure to cooling towers, whirlpool spas, and decorative fountains.
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Pneumonia Information

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