The most common causes of pneumonia are infections caused by:
- bacteria - the most common cause of pneumonia in adults
- viruses - often responsible for pneumonia in children
- mycoplasma - organisms that have characteristics of bacteria and viruses that cause milder infections
- opportunistic organisms - a threat to people with vulnerable immune systems (e.g., Pneumocystis carinii pneumonia in people who have AIDS)
Most types of pneumonia are transmitted in the same way as influenza or the common cold - by people's hands and by tiny droplets from their mouths and noses. In fact, the same viruses that cause colds and the flu can cause pneumonia. If they infect the throat, sinuses, and upper respiratory tract, they cause a cold. If they reach the lungs, they cause pneumonia.
Bacteria that live permanently in many peoples' throats cause some of the more severe types of pneumonia. Normally, the immune system keeps them in check. If someone is weakened by a throat virus, these bacteria can trickle down into the respiratory tract. Bacterial pneumonia is most often caused by the bacteria Streptococcus pneumoniae (pneumococcus).
Respiratory syncytial virus (RSV) is the most common cause of pneumonia in infants and young children. It peaks sharply around December and January and usually isn't a life-threatening disease, though some individuals can be seriously affected. Viruses cause about half of all cases of pneumonia.
Mycoplasma causes the illness called "walking pneumonia," so-called because people who have it are not confined to bed.
A fungus called Pneumocystis carinii is usually seen only in people who have AIDS. This parasite is normally harmless, but in people with HIV it can cause an aggressive and often fatal pneumonia.
In addition to infectious diseases, people can get pneumonia from chemicals that enter the lungs and inflame them. Aspiration pneumonia is caused by accidentally inhaling food, vomit, or digestive acid into the lungs. The inhaled substance may become infected, or it may inflame the lungs and cause them to fill with liquid.
A person who has a higher risk of pneumonia:
- is under one year of age or over the age of 65
- is a smoker
- has a cold or flu
- has a weak immune system due to cancer therapy, HIV infection, or other disease
- is undergoing surgery
- has a problem with alcohol use
- has a chronic illness such as heart disease, lung disease, or diabetes
- has a chronic lung disease, such as asthma or chronic obstructive pulmonary disease