Americans Decrease Increasingly, historians are blaming diseases imported from the Old World for the staggering disparity(不同) between the indigenous (本土的) population of America in 1492—new estimates of which soar 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 nineth century. There is no doubt that chronic disease was an important factor in the precipitous(险峻的) 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(免疫学的) almost defenseless. That virgin-soil epidemics were important in American history is strongly indicated by evidence that a number of erous maladies(病)—small pox, measles(麻疹) ,malaria(疟病), 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 horrendous (可怕的) epidemics and steep population declines, confirmed in many cases by recent equantitative yses 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 begin to keep continuous records until the sieth century, by which time the worst epidemics had probably already taken place. Furthermore , the British tended to drive the native populations away, rather than enslaving 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 indigenous population. In 1616—1619 an epidemic, possibly of bubonic or pneumonic (肺的) plague (瘟疫), swept coastal New England, killing as many as nine out of ten. During the 1630’’s small-pox, the disease most fatal to the Native American people, eliminated half the population of the Huron and Iroquois confederations. In the 1820’’s fever devastated the people of the Coulumbia 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 do 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 devastating consequences when they strike an immunologically defenseless community. The author implies which of the following statements about measles