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    细菌和病毒都是可以致病的微生物,但它们的特征区别很大。细菌虽然小,要在光学显微镜下才能看得见,但它除了拥有生命的基本单位核酸之外,还有一大套赖以生存的配套设施。包括作为居住“公馆”的细胞壁,储存营养物质的“仓库”,以及进行新陈代谢的“化工车间”。依靠这些,细菌能够摄取外界的物质并加工成需要的能量。

    而病毒就更小了,也可怜得多,且不奢谈“库房”和“车间”,就连作为保护外壳的“茅草房”也没有。实际上它只有一个分子大小,用电子显微镜才能看得见。整个家当也只是一条表示生命的核酸而已。如果拿人来做比喻,细菌最起码也有条裤子,有只讨饭碗,有根打狗棒。所以细菌虽然必须在人体内部的良好环境中才能繁殖,但处在恶劣环境中仍能生存较长的一段时间。而病毒则像个刚出生的婴儿,除了它的生命和一张吃奶的嘴外便一无所有,毫无独立生存的能力。因而病毒只能寄生在人或动物的细胞内部,靠“窃取”细胞里的现成营养才能生存。一旦被排出体外,病毒就活不了几小时。

    大部分抗生素对细菌起作用,是因为抗生素可以抑制细胞繁殖,干扰它们形成新的遗传结构或者细胞壁。而正因为病毒只能寄生在别人的细胞内,自己不能完成这些生化反应,所以抗生素对病毒全无作用。病毒的生存能力既然这么弱,为什么还会那样猖獗呢?例如埃博拉的感染力极强,病死率可达80%。实际上大多数病毒远没有那么可怕,有的也不会使人得病。冠状病毒本来致病的能力并不强,问题在于“变种”上。变异的病毒和原来的不同了,它可能是无害的,但也可能变成“杀伤力”更大的病毒。然而我们不必担心它会因反复变异而使杀伤力次第增大。因为就杀伤力而言,变异就像赌博,总是有输有赢,不断地赢下去的几率是微乎其微的。至少有史以来还没有过这样的记录。如果有过,可能现在就没有我们的存在了。

    必须指出的是杀伤力的大小不仅取决于病毒一方,更重要的是人群的免疫力。变种冠状病毒之所以为患甚大,很重要的一个因素是人类还没有接触过这样变种后的新病毒。当人群有足够多的人产生了抗体之后,这种病毒对人类的威胁也就小得多了。

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Directions: For each blank in the following passage there are four words or phrases marked A, B, C and D. Fill in each blank with the word or phrase that best fits the context.

           If you studied pictures that ancient people left on rock walls and you tried to determine their meaning, you would not detect interest in romance among the artists. 1, you would see plenty of animals with people running after them. Life for ancient people’s earned to center on hunting and gathering wild foods for meals.

           In modern times, when food is available in grocery stores, finding love is more 2 in people’s lives. The 3is all around us. It is easy to prepare a list of modern stories having to do with love. An endless number of books and movies qualify as love stories in popular culture.

           Researchers are studying whether love, a highly valued emotional state, can be 4. They ask, what is love? Toothpaste companies want us to think attraction is all about clean teeth, but clean teeth go only so far. Scientists wonder how much the brain gets involved. You have probably heard that opposites attract but that 5attract, too. One thing is certain: The truth about love is not yet set in stone.

First Impression

           To help determine the 6 of attraction, researchers paired 164 college classmates and had them talk for 3, 6 or 10 minutes so they could get a sense of each other’s individuality. Then students were asked to7what kind of relationship they were likely to build with their partners. After nine weeks, they reported what happened.

As it turned out, their 8judgments often held true. Students seemed to9at an early stage who would best fit into their lives.

The 10 Knows

           Scientists have also turned to nonhumans to increase understanding of attraction. Many animals give off pheromones — natural chemicals that can be detected by, and then can produce a response in, other animals of the same species. Pheromones can signal that an animal is either ready to fight or is feeling 11 to partnerships. In contrast, humans do not seem to be as 12 as other animals at detecting such chemicals. Smell, however, does seem to play a part in human attraction. Although we may not be aware of chemicals like pheromones consciously, we give and receive loads of information through smell in every interaction with other people.

Face Value

           Being fond of someone seems to have a number of factors, including seeing something we find attractive. Researchers had people judge faces for 13. The participants had 0.013 seconds to view each face, yet somehow they generally considered the images the same as people who had more time to study the same faces. The way we14attractiveness seem to be somewhat automatic.

When shown an attractive face and then words with good or bad associations, people responded to 15words faster after viewing an attractive face. Seeing something attractive seems to cause happy thinking.