资料题：The author' s main purpose is to（  ）。

Pneumonia, an acute inflammation of the lungs, is not a single disease but more like a family of several dozen diseases, each caused by a different agent.The agents include a variety of bacteria, viruses, fungi, parasites and chemicals and produce different symptoms, but typically patients have fever, difficulty breathing, chest pain and coughing including the coughing up of blood.The symptoms last a week or more, and in its classic form, lobar pneumonia, 30 percent of patients die if not treated.Transmission is usually by inhalation (呼吸) but also by hand-to-mouth contact.Patients in hospitals, where pathogens(病原体)exist in great numbers, are vulnerable.The immune system, the mechanical action o[ coughing and the microscopic motion of cilia(睫、纤毛)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 (损害)by, say, AIDS or cancer, are also at high risk, s 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.There is an ancient theory, suggested by the higher mortality rates (死亡率) documented in winter, that cold temperatures promote pneumonia, but this idea is not correct: Massachusetts has a high rate, but 80 does Georgia, and North Dakota has the third lowest rate.Florida is lowest, which may reflect the"healthy retiree" effect; that is, the tendency of healthy older people to retire to places like Florida while the less healthy remain home.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 (希波拉底式的、著名的) physicians of ancientGreece.in 1900 it was the second deadliest killer in the U.S.after tuberculosis.The extraordinarily high rate in 1918 resulted from the great influenza pandemic that year, which killed more than 540,000 Americans.Since then, pneumonia mortality rates have decreased markedly because of better hygiene and increasingly effective methods of treatment: first, anti-pneumococcal serum(血清), then sulfa drugs, and finally, in the 1940s, penicillin.