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  3. (1) A year ago, most people would have drawn a blank if asked about Zika. Since then, an outbreak of the mosquito-borne virus th

(1) A year ago, most people would have drawn a blank if asked about Zika. Since then, an outbreak of the mosquito-borne virus th

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问题     (1) A year ago, most people would have drawn a blank if asked about Zika. Since then, an outbreak of the mosquito-borne virus that began in early 2015 in Brazil has spread to more than 60 countries in the Americas, Africa, Asia and the Pacific islands. A study published on September 1st in the Lancet estimates that 2.6 billion people live in areas to which Zika could eventually spread.
    (2) At first, scientists knew little more than anyone else. Zika is not new; the virus was first isolated in Africa in 1947. But it was obscure, and therefore little studied. Only during the present outbreak did it become clear that infection among pregnant women was associated with birth defects and neurological problems in babies. But there has been much progress, and scientists now know far more about the disease than they did when the outbreak began.
    (3) Start with transmission. The vast majority of Zika infections occur through the bite of Aedes aegypti, a mosquito common in tropical climates and especially in cities. Another species, A. albopictus, which thrives in cooler climes, may also be able to transmit the bug, though possibly not as efficiently. Unusually for a mosquito-borne virus, Zika can also be transmitted sexually (the first case of transmission in the United States occurred this way). Studies are under way to find out how long after infection that remains possible, but traces of the virus’s genetic code have been found in semen six months after the onset of symptoms. Infection through blood transfusion has been confirmed as well. The virus has also turned up in urine, tears and saliva, though that does not necessarily mean that it can spread through them.
    (4) The health effects of the virus are becoming clearer too. Something like four in five Zika infections cause no symptoms. The rest usually pass with only mild discomfort, including a rash and red eyes. Occasionally, infected people develop Guillain-Barre syndrome, a condition in which the immune system goes awry, causing weakened muscles and temporary paralysis. Death is rare, but some sufferers spend weeks hooked to a breathing machine.
    (5) Infection is also dangerous if it occurs during pregnancy: in perhaps 1％ - 2％ of cases the virus attacks the brain tissue of the fetus. That causes microcephaly, a condition characterised by an abnormally small head, a result of the skull collapsing around the shrunken brain. Babies who escape that fate may suffer other Zika-related damage, including eyesight and hearing loss. Scans of apparently healthy babies born to infected mothers sometimes show brain abnormalities, though it is too early to know whether these will lead to developmental problems later in life. And there are worries, as yet unresolved, about the neurological implications in adults, too.
    (6) Then there is the question of tracking and diagnosis. Working out just how far Zika has spread within a country is tricky. A common test works by testing for antibodies, specialised proteins produced by the immune system that are designed to disable the virus. But it cannot distinguish easily between antibodies for Zika and those for dengue fever, another mosquito-borne illness, which is related to Zika and often occurs in the same sorts of places. That may turn out to be a good thing: antibodies against dengue may provide some defence against Zika. But it muddles attempts to track the disease, and to predict how it might spread.
    (7) Two open questions are whether a Zika infection confers lasting immunity to the virus, and how strains from the two known lineages—one African and one Asian—might interact. There are reasons to worry: an initial infection with one of the four strains of dengue is usually harmless, but subsequent infection with another strain can be fatal.
    (8) Official advice continues to evolve with the stream of new findings. Preventing mosquito bites is the main line of defence. The World Health Organisation prescribes condoms or sexual abstinence for at least six months for those returning from areas where Zika is spreading. Several countries have begun screening blood donors.
    (9) The most encouraging news is on the vaccine front. Several are in early-stage trials. Two—one developed by the National Institutes of Health in America, and the other by Inovio Pharmaceuticals, a private firm—use a new technology called "DNA vaccination". Traditional vaccines use either dead viruses or weakened live ones to provoke an immune system. DNA is much easier to handle than weakened or dead viruses; and by focusing on genetic sequences common to different variants, a vaccine may offer protection against several strains of the virus. (本文选自 The Economist)
What can be learned from Para. 4?
选项 A、Zika infections are always fatal.
B、Zika infections show no symptoms.
C、Zika infections may show mild symptoms.
D、Zika infections always cause serious symptoms.
答案C
解析 推断题。原文第四段提到，大约五分之四的寨卡感染不会出现任何症状，即使有症状也只是诸如皮疹、红眼病等轻微症状，偶尔会有肌肉无力和暂时性麻痹的现象，致死率很低，故C为答案，同时可排除A、B和D。
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