Your body has ways to protect your lungs from infection. In fact, you're frequently exposed to bacteria and viruses that can cause pneumonia, but your body normally uses a number of defenses, such as cough and the normal microorganisms (flora) in your body, to prevent harmful organisms from invading and overwhelming your airways. However, numerous conditions, including malnutrition and systemic illness, can lower your protection and allow harmful organisms to get past your body's defenses and into your lungs.
Once the invading organisms are in your lungs, white blood cells — a key part of your immune system — begin to attack them. The accumulating invaders, white blood cells and immune system proteins cause the tiny air sacs in your lungs to become inflamed and filled with fluid, leading to the difficult breathing that characterizes many types of pneumonia.
Classifications of pneumonia
Pneumonia is sometimes classified according to the cause of pneumonia:
• Community-acquired pneumonia. This refers to pneumonia you acquire in the course of your daily life — at school, work or the gym, for instance. The most common cause is the bacterium Streptococcus pneumoniae. Another, less common cause is Mycoplasma pneumoniae, a tiny organism that typically produces milder signs and symptoms than other types of pneumonia. Walking pneumonia, a term used to describe pneumonia that isn't severe enough to require bed rest, may result from Mycoplasma pneumoniae.
• Hospital-acquired (nosocomial) pneumonia. If you're hospitalized, you're at a higher risk of pneumonia, especially if you're breathing with the help of a mechanical ventilator, in an intensive care unit or have a weakened immune system. This type of pneumonia can be extremely serious, especially for older adults, young children and people with chronic obstructive pulmonary diseases (COPD) or HIV/AIDS.
Hospital-acquired pneumonia develops at least 48 hours after you're admitted to the hospital. This category includes post-operative pneumonia — most common in people older than age 70 who have abdominal or chest surgery — and health-care associated pneumonia — acquired in chronic care facilities, centers where drugs are given by intravenous drip (infusion) and kidney dialysis centers.
A common predisposing factor for this type of pneumonia is gastroesophageal reflux disease (GERD). This occurs when some of the contents of your stomach flow back into the upper esophagus. From there, the gastroesophageal contents can be inhaled (aspirated) into your windpipe and then into your lower airways. Even small amounts of gastroesophageal reflux can lead to pneumonia in people who are hospitalized.
• Aspiration pneumonia. This type of pneumonia occurs when you aspirate foreign matter into your lungs — most often when the contents of your stomach enter your lungs after you vomit. This commonly happens when a brain injury or other condition affects your normal gag reflex.
Another cause of aspiration pneumonia is consuming too much alcohol. Aspiration occurs when the inebriated person passes out and then vomits due to the effects of alcohol on the stomach. If someone's unconscious, it's possible to aspirate the liquid contents and possibly solid food from the stomach into the lungs, causing aspiration pneumonia.
Difficulty swallowing, which occurs with diseases such as amyotrophic lateral sclerosis (ALS), Parkinson's disease and strokes, may also lead to aspiration pneumonia.
• Pneumonia caused by opportunistic organisms. This type of pneumonia strikes people with weakened immune systems. Organisms that aren't harmful for healthy people can be dangerous for people with AIDS and other conditions that impair the immune system, as well as people who have had an organ transplant. Medications that suppress your immune system, such as corticosteroids or chemotherapy, also can put you at risk of opportunistic pneumonia.
• Other pathogens. Outbreaks of the H5N1 influenza (bird flu) virus and severe acute respiratory syndrome (SARS) have caused serious, sometimes deadly pneumonia infections, even in otherwise healthy people. Although rare, anthrax, plague and tularemia also may cause pneumonia. Some forms of fungi, when inhaled can cause pneumonia. Tuberculosis in the lung also can cause pneumonia.