Historically, the spread, prevalence, and very existence of contagious disease have wholly depended on the growth and concentration of human populations.（1） And though the last century has witnessed substantial worldwide success in combating many past scourges —such as polio and smallpox — infectious diseases still claim more lives than any other group of diseases. The prevailing demographic trends continue to create a crowded human “medium” that both invites and is vulnerable to infection.The share of humanity living in cities with more than 1 million people has surged from less than 5 percent in 1900 to nearly 40 percent today, creating the ideal setting for the resurgence of old infectious diseases as well as the development of new ones.（2）Overcrowding — the increased proximity of susceptible individuals — is a principal risk factor for the incidence and spread of all major infectious diseases, including tuberculosis, dengue fever, malaria, and acute respiratory illnesses, which are unable to spread and survive in low population densities.（3）Aside from sheer growth and increasing density, the urbanization under way in developing nations is often accompanied by deteriorating health indicators and increased exposure to disease risk factors.Access to clean water, good hygiene, and adequate housing are sorely lacking in developing nations. As a result, waterborne infections such as cholera and other diarrheal diseases account for 90 percent of all infectious diseases in developing countries — and 40 percent of all deaths in some nations.（4）In both industrial and developing nations, the incidences of a wide range of infectious diseases, including tuberculosis, diarrheal diseases, and HIV/AIDS, are considerably higher in urban slums — where poverty and compromised health define the way of life — than in the rest of the city.（5 ）

A.Key disease carriers, such as insects and rats, thrive in crowded urban settings, further facilitating spread. B.The unprecedented population densities in fourteenth-century Europe, for example, led to the plague outbreak that claimed the lives of one fourth of the population. C.Although these infections are easily preventable if adequate water and sanitation are available, the vast majority of the world’s population are lifelong victims. D.While new global markets have created unprecedented economic opportunities and growth, the health risks of our increasingly interconnected and fast-paced world continue to grow. E.Pathogens can more readily establish in large populations, since all infectious diseases require a critical number of vulnerable individuals in order to take root and spread. F.These areas can serve as a perpetual reservoir of disease or disease vectors, placing other parts of the city at risk of an outbreak and allowing the disease to continue evolving, often into a deadlier strain.
问题2：
A.Key disease carriers, such as insects and rats, thrive in crowded urban settings, further facilitating spread. B.The unprecedented population densities in fourteenth-century Europe, for example, led to the plague outbreak that claimed the lives of one fourth of the population. C.Although these infections are easily preventable if adequate water and sanitation are available, the vast majority of the world’s population are lifelong victims. D.While new global markets have created unprecedented economic opportunities and growth, the health risks of our increasingly interconnected and fast-paced world continue to grow. E.Pathogens can more readily establish in large populations, since all infectious diseases require a critical number of vulnerable individuals in order to take root and spread. F.These areas can serve as a perpetual reservoir of disease or disease vectors, placing other parts of the city at risk of an outbreak and allowing the disease to continue evolving, often into a deadlier strain.
问题3：
A.Key disease carriers, such as insects and rats, thrive in crowded urban settings, further facilitating spread. B.The unprecedented population densities in fourteenth-century Europe, for