单选题
It was the kind of research that gave insight into how flu strains could mutate so quickly. (One theory behind the 1918 version's sudden demise after wreaking so much devastation was that it mutated to a nonlethal form. ) The same branch of research concluded in 2005 that the 1918 flu started in birds before passing to humans. Parsing this animal-human【C1】______could provide clues to【C2】______the next potential superflu, which already has a name: H5N1, also known as avian flu or bird flu. This potential killer also has a number: 59 percent. According to the World Health Organization, nearly three-fifths of the people who【C3】______H5N1 since 2003 died from the virus, which was first reported【C4】______humans in Hong Kong in 1997 before a more serious【C5】______occurred in Southeast Asia between 2003 and 2004. (It has since spread to Africa and Europe. ) Some researchers argue that those mortality numbers are exaggerated because WHO only【C6】______cases in which victims are sick enough to go to the hospital for treatment【C7】______compare that to the worldwide mortality rate of the 1918 pandemic; it may have killed roughly 50 million people, but that was only 10 percent of the number of people infected, according to a 2006 estimate. H5N1's saving grace — and the only reason we're not running around masked up in public right now — is that the strain doesn't jump from birds to humans, or from humans to humans, easily. There have been just over 600 cases (and 359 deaths) since 2003. But【C8】______its lethality, and the chance it could turn into something far more transmissible, one might expect H5N1 research to be exploding, with labs【C9】______the virus's molecular components to understand how it spreads between animals and【C10】______to humans, and hoping to discover a vaccine that could head off a pandemic.