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Community acquired pneumonia is spread by close person-to-person contact—usually when an infected person coughs or sneezes on another person. CAP can be caused by several different organisms, including bacteria, viruses, and fungi. The most common organism responsible for CAP is the bacterium known as Streptococcus pneumoniae. Although several "bugs" or organisms have been confirmed to be causes of CAP, about 30% to 50% of pneumonia cases are reported to have an unknown cause—meaning the exact "bug" responsible for the infection is unknown or is not identified via laboratory testing.
Bacterial pneumonias tend to be the most serious and, in adults, the most common cause, especially Streptococcus pneumoniae (pneumococcus). Respiratory viruses are the most common causes of pneumonia in young children, peaking between the ages of 2 and 3. By school age, the bacterium Mycoplasma pneumoniae becomes more common.
In some people, particularly the elderly and those who are debilitated, bacterial pneumonia may follow influenza or even a common cold. Many people contract pneumonia while staying in a hospital for other conditions. This tends to be more serious because the patient's immune system is often impaired due to the condition that initially required treatment. In addition, there is a greater possibility of infection with bacteria that are resistant to antibiotics.
Risk factors are characteristics that may increase the chance for developing a condition. The more risk factors present, the more likely you are to develop the condition. You are at an increased risk for developing CAP if you:
- are 65 years or older
- have other medical conditions or a combination of conditions such as:
- chronic obstructive lung disease (COPD) or other chronic lung disorders
- diabetes mellitus
- chronic kidney disease
- heart failure
- coronary artery disease
- cancer
- chronic liver disease
- cystic fibrosis
- are a smoker
- are exposed to certain chemicals or pollutants such as those used for agriculture, construction, or industrial chemicals. Exposure to these pollutants can sometimes cause damage to the lungs and contribute to lung inflammation—thus leaving the lungs more susceptible to infection.
- suffer from alcoholism
- have a weakened immune system
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