[单项选择]The Spread of Microorganisms 　　There is scarcely a place on earth which is naturally free of microorganisms 　　(46). But on external body surfaces, on lining membranes and inside tubes and organs that are connected with the outside---such as the mouth, throat, nose and large intestine ---- both harmless and pathogenic microbes live in large numbers. (47). If these natural defenses are sound, a person may conceal many microbes without ill effect. 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 breaking out. 　　(49). 　　Although microbes cannot fly, the dust of the air is alive with them. In close quarters the atmosphere is further polluted by bacteria-loaded tiny drops discharged by coughing and even normal conversation. Pathogenic microbes are also spread by rats, mice, flies and mosquitoes. (50). In slum areas there