Pneumonia is an inflammation of the lungs that is usually caused by infection. Pneumonia can also be caused by inhaling irritants such as vomit, liquids, or chemicals. With pneumonia, the air sacs in the lungs fill with liquid or pus, which interferes with the lungs' ability to transfer oxygen to the blood.
Before the invention of antibiotics in the 1930s, pneumonia was a leading cause of death. Though it has since become very treatable, pneumonia remains a public health problem.
There are many different kinds of pneumonia, ranging from mild to severe. There are 4 basic types:
- Community-acquired pneumonia (CAP), the most common type of pneumonia, is caused by bacteria, viruses, and other organisms that are acquired outside of the hospital or other health care settings.
- Hospital-acquired pneumonia (HAP) occurs at least 48 hours after someone has been admitted to the hospital. It can be caused by bacteria and other organisms that are usually different from CAP. HAP is usually more serious than CAP because the bacteria and organisms can be harder to treat, and because people who get HAP are already sick.
- Aspiration pneumonia occurs when liquids or other irritants are inhaled into the lungs. The most common type of aspiration pneumonia is caused by inhaling stomach contents after vomiting. People with medical problems (e.g., stroke, ALS) that affect swallowing are at an increased risk of this type of pneumonia.
- Opportunistic pneumonia occurs in people with weakened immune system (e.g., people with AIDS, cancer, organ transplant). Organisms that are not usually harmful to people with healthy immune systems cause these types of infections.
Pneumonia rarely causes serious complications for healthy people under 65 years of age. People who suffer from chronic respiratory diseases or who have compromised immune systems are generally at greater risk for developing pneumonia.