Increasingly, historians are blaming diseases imported from the Old World for the great disparity between the native population of America in 1492--new estimates of which jump as high as 100 million, or approximately one-sixth of the human race at that time--and the few million full-blooded Native Americans alive at the end of the nineteenth century. There is no doubt that chronic disease was an important factor in the sharp decline, and it is highly probable that the greatest killer was epidemic disease, especially as manifested in virgin-soil epidemics.

Virgin-soil epidemics are those in which the populations at risk have had no previous contact with the diseases that strike them and are therefore immunologically almost defenseless. That virgin-soil epidemics were important in American history is ply indicated by evidence that a number of dangerous maladies--smallpox, measles, malaria, yellow fever, and undoubtedly several more--were unknown in the pre-Columbian New World. The effects of their sudden introduction are demonstrated in the early chronicles of America, which contain reports of horrible epidemics and steep population declines, confirmed in many cases by quantitative analyzes of Spanish tribute records and other sources. The evidence provided by the documents of British and French colonies is not as definitive because the conquerors of those areas did not establish permanent settlements and began to keep continuous records until the seventeenth century, by which time the worst epidemics had probably already taken place. Furthermore, the British tended to drive the native populations away, rather than to enslave them as the Spaniards did; so that the epidemics of British America occurred beyond the range of colonists’ direct observation.

Even so, the surviving records of North America do contain references to deadly epidemics among the native population. In 1616--1619 an epidemic, possibly of pneumonic plague, swept coastal New England, killing as many as nine out of ten. During the 1630’s smallpox, the disease most fatal to the Native American people, eliminated half the population of the Huron and Iroquois confederations. In the 1820’s fever ruined the people of the Columbia River area, killing eight out of ten of them.

Unfortunately, the documentation of these and other epidemics is slight and frequently unreliable, and it is necessary to supplement what little we de know with evidence from recent epidemics among Native Americans. For example, in 1952 an outbreak of measles among the Native American inhabitants of Ungava Bay, Quebec, affected 99 percent of the population and killed 7 percent, even though some had the benefit of modern medicine. Cases such as this demonstrate that even diseases that are not normally fatal can have destroying consequences when they strike an immunologically defenseless community.

Notes: disparity 差距。 virgin-soil处女地。 malady 疾病 chronicle 编年史。 tribute 贡品。 pneumonic plague肺鼠疫。confederation 同盟。 smallpox 天花。measles 麻疹。

The primary purpose of the text is to（）

A. provide support for a hypothesis

B. reconcile opposing viewpoints

C. refute a common misconception

D. analyze an argument in detail