The most rampantly infectious organism on Earth, a bacterium called Wolbachia, doesn't hurt humans at all—or, come to that, any other vertebrates—but if you are a shrimp or worm or fruit fly, it can make you wish you had never been born. Altogether, only about one microbe in a thousand is a pathogen for humans, according to National Geographic —though, knowing what some of them can do, we could be forgiven for thinking that that is quite enough. Even if mostly benign, microbes are still the number-three killer in the Western world, and even many less lethal ones of course make us deeply rue their existence.
地球上最具传染性的生物，一种名叫沃尔巴克体的细菌，根本有伤害人类，或者可以说根本不伤害任何别的脊椎动物——不过，要是你是个小虾、蠕虫或果蝇，你会但愿自己真没有被生出来。据《国家地埋杂志》说，总的来说，大约每1000种微生物当中，只有一种是对人类致病的——虽然我们知道其中还会有一些干坏事，情有可原地这么认为就够了。即使大多数生物是无害的，微生物仍是西方世界的第三杀手——虽然许多不要我们的命，但也弄得我们深深地后悔来到这个世界上。

Making a host unwell has certain benefits for the microbe. The symptoms of an illness often help to spread the disease. Vomiting, sneezing, and diarrhea are excellent methods of getting out of one host and into position for another. The most effective strategy of all is to enlist the help of a mobile third party. Infectious organisms love mosquitoes because the mosquito's sting delivers them directly to a bloodstream where they can get straight to work before the victim's defense mechanisms can figure out what's hit them. This is why so many grade-A diseases—malaria, yellow fever, dengue fever, encephalitis, and a hundred or so other less celebrated but often rapacious maladies—begin with a mosquito bite. It is a fortunate fluke for us that HIV, the AIDS agent, isn't among them—at least not yet.
把寄主弄得很不舒服，对微生物是有某种好处的。病症往往有利于传播细菌。呕吐、打喷嚏和腹泻是细菌离开一个寄主，准备入住另一寄主的好办法。最有效的方法是找个移动的第三者帮忙。传染性微生物喜欢蚊子，因为蚊子的螫针可以把它们直接送进流动的血液，趁受害者的防御系统尚未搞清受到什么攻击之前，它们可以马上着手干活。因此，许多A级疾病——疟疾、黄热病、登革热、脑炎，以及100多种其他不大著名而往往又很严重的疾病——都是以被蚊子叮咬开始的。对我们来说，很侥幸的是，艾滋病的介体——人体免疫缺陷病毒——不在其中，至少目前还不在其中。