单项选择题

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 is often a combination of crowded conditions and poor sanitation. In addition, many of the inhabitants have lowered resistance because of shortage of food and other undesirable unhealthy practices. As a result, epidemics are likely to begin in these districts. 　　All civilized societies establish and enforce measures designed to protect the health of their populations. Most of these practices are concerned with preventing the spread of the pathogenic organisms. 　　A. The atmosphere is a carrier of microorganisms. 　　B. One exception is the interior of normal body tissue. 　　C. Microbial growth is further helped by the widespread presence of dirt and the lack of sunlight. 　　D. The body has natural defenses against these organisms. 　　E. The body has no defenses against these organisms. 　　F. The simplest way is by person-to-person contact.