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Pneumonia is caused by viruses, bacteria, or (in rare cases) parasites or other organisms. In up to 65% of cases, the organism (such as bacteria or virus) that is causing pneumonia is not identified even with testing. In adults, pneumonia is most often caused by bacteria such as Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenzae, and Legionella. The Legionella bacteria was responsible for a well-known outbreak of pneumonia called Legionnaires’ disease. Young children are more likely to develop pneumonia from exposure to a virus, such as the parainfluenza and influenza viruses, respiratory syncytial virus, and adenovirus. The chickenpox virus can cause pneumonia in adults and children. Mycoplasma pneumoniae is the cause of walking pneumonia, more likely to occur in older children and younger adults. Bird droppings, specifically poultry, carry an organism called Chlamydia psittaci, which can also cause pneumonia. Pneumocystis carinii, which has been classified as a parasite and a fungi, causes pneumonia in people with compromised immune systems, such as those with AIDS or undergoing cancer treatment. Healthy lungs are free of any type of bacteria or virus because our body has many safeguards that protect the lungs. However, these safeguards can be overwhelmed by exposure to a large number of the organisms that cause pneumonia.
Pneumonia usually starts when a person inhales infected air particles into the lungs. In other cases, it develops during or after a viral upper respiratory infection, such as a cold or the flu. Pneumonia also may occur as a complication of a viral illness such as measles or chickenpox. Pneumonia can develop if a person inhales food, vomit, or mucus into the lungs (aspiration pneumonia). Most cases of pneumonia are contracted by breathing in small droplets that contain the bacteria or virus that can cause pneumonia. These droplets get into the air when a person infected with these germs coughs or sneezes. In other cases, pneumonia is caused when bacteria or viruses that are normally present in the mouth, throat, or nose inadvertently enter the lung. During sleep it is quite common for people to aspirate secretions from the mouth, throat, or nose. Normally, the body's reflex response (coughing back up the secretions) and immune system will prevent a pneumonia from starting. However, if a person is in a weakened condition from another illness, a severe pneumonia can develop. People with emphysema, heart disease, and swallowing problems, as well as alcoholics, drug users and those who have suffered a stroke or seizure are at higher risk for developing pneumonia.
Once the bacteria, virus or fungus enter the lungs, they usually settle in the air sacs of the lung where they rapidly grow in number. This area of the lung then becomes filled with fluid and pus as the body attempts to fight off the infection. 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.
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