Patients in hospitals, which abound with pathogens, are vulnerable, especially through invasive devices such as catheters and respirators. The immune system, the mechanical action of coughing and the microscopic motion of cilia normally protect healthy individuals. But old people, who generally have weaker defense mechanisms than the young, are far more likely to die of pneumonia. Those whose defenses are compromised by, say, AIDS or cancer, are also at high risk, as are those given certain medicines such as immunosuppressive drugs. Men are at higher risk than women, partly because they are more prone to alcoholism and nicotine addiction, two of the many risk factors for pneumonia.
Blacks are at higher risk than whites, perhaps because they often lack access to good medical care.
Air pollution also plays a role. In recent years, pneumonia was the underlying cause of death for about 80,000 Americans annually, whereas for an additional 100,000 or so, it was a contributing cause of death. The map shows only those deaths for which pneumonia is registered as the underlying cause. California has the highest rate, perhaps in part because of air pollution levels in southern California. Pneumonia probably affected prehistoric humans and is one of the oldest diagnosed diseases, having been described by the Hippocratic physicians of ancient Greece.