2010年职称英语理工类B级补全短文考点精华(3)

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　　TheSpread of Microorganisms
　　There is scarcely a place on earth which isnaturally free of microorganisms (微生物). 46 . But on external body surfaces, onlining membranes(内膜) and inside tubes and organs that are connected with theoutside—such as the mouth, throat, nose, and large intestine(肠)—bothharmless and pathogenic(病原的)microbes(微生物)live in large numbers. 47 . If thesenatural defenses are sound, a person may conceal many microbes without illeffect. However, if his resistance becomes lowered, an infection can result.
　　Microbes are spread about through an almost infinite variety of means. 48. The more crowded the conditions, the greater the chances of epidemics(流行病)breaking out. 49 .