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  3. According to the "hygiene hypothesis," first proposed in 1989, exposure to a variety of bacteria, viruses and parasitic worms ea

According to the "hygiene hypothesis," first proposed in 1989, exposure to a variety of bacteria, viruses and parasitic worms ea

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问题     According to the "hygiene hypothesis," first proposed in 1989, exposure to a variety of bacteria, viruses and parasitic worms early in life helps prime a child’s immune system, much like sensory experiences program his brain. Without such early instruction, the immune system may go in disorder and overreact with allergies to foods, pollen and pet fur or turn on the body’s own tissue, setting off autoimmune disorders.
    Many of these microorganisms evolved symbiotically(共生地)with humans over millions of years— the so-called "old friends" theory. But where they’ve been eradicated, a key part of human development has been thrown off. "The vast majority of microbes are harmless. There are only a few dozen that can cause lethal infections," says Thomas McDade, director of the Laboratory for Human Biology Research at Northwestern University.
    In 1998, about one in five children in industrialized countries suffered from allergic diseases such as asthma(哮喘), allergies and rashes, according to the International Study of Asthma and Allergies in Childhood, a global research initiative. The incidence of peanut allergy in the US tripled between 1997 and 2008, according to a report from Mount Sinai School of Medicine in New York. But such diseases are still relatively rare in Africa and rural Asia. "The geographical distribution of allergic and autoimmune diseases is a mirror image of the geographical distribution of various infectious diseases," says a report by French researchers devoted to the hygiene hypothesis.
    Exposure to immune-stimulating germs may also lower the risk of heart disease, according to Dr. McDade. But there are other dangers lurking in muddy water and animal feces. Michael Bell, an infectious disease specialist and deputy director of Healthcare Quality Promotion at the Centres for Disease Control and Prevention, says that people should be vigilant about wound care since bacteria can cause problems if they get into the blood stream, and he still advocates hand-washing. "If you’re not doing it ten times a day, you’re probably not doing it enough," he says. But he and other experts say that regular soap and water arc fine in most cases. Sterilizing hands is critical mainly for health-care workers and in hospitals, where disease-causing germs are prevalent and can easily spread.
    Many experts advise common sense. "We don’t want to say to children, OK, play by the dirty river bank and catch whatever you can,"says Dr. Weinstock. "But we can say there’s nothing wrong with kids playing in the dirt. They don’t have to live in total sanitation(环境卫生), and they won’t die from eating something off the floor. It’s probably more healthy than not."
The "hygiene hypothesis" proposed that exposure to various viruses and germs in early childhood______.
选项 A、eliminates childhood diseases such as asthma and allergies
B、proves to be an important prevention against cancer
C、proves to be necessary for the development of immune system
D、increases susceptibility to allergic and autoimmune diseases
答案C
解析
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