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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(內(nèi)膜) and inside tubes and organs that are connected with theoutside�C—�Csuch as the mouth, throat, nose, and large intestine(腸)�C—�Cbothharmless 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 .

　　Although microbes cannot fly, the dust of the air isalive with them. In close quarters the atmosphere is further polluted bybacteria-loaded tiny drops discharged by coughing and even normal conversation.Pathogenic microbes are also spread by rats, mice, flies and mosquitoes. 50 . Inslum areas there is often a combination of crowded conditions and poorsanitation(環(huán)境衛(wèi)生). In addition, many of the inhabitants have lowered resistancebecause of shortage of food and other undesirable unhealthy practices. As aresult, epidemics are likely to begin in these districts.

　　All civilizedsocieties establish and enforce measures designed to protect the health of theirpopulations. Most of these practices are concerned with preventing the spread ofthe pathogenic organisms

　　A The atmosphere is a carrier ofmicroorganisms

　　B One exception is the interior of normal bodytissue

　　C Microbial growth is further helped by the widespread presence ofdirt and the lack of sunlight

　　D The body has natural defenses againstthese organisms

　　E The body has no defenses against these organisms

　　FThe simplest way is by person-to-person contact