单项选择题

Passage 2

In 1957 a doctor in Singapore noticed that hospitals were treating an unusual number of influenzalike cases. Influenza is sometimes called "flu" or a "bad cold". He took samples from the throats of patients and in his hospital was able to find the virus of this influenza.
There are three main types of influenza virus. The most important of these are types A and B, each of them having several sub-groups. With the instruments at the hospital the doctor recognized that the outbreak was due to a virus group A, but he didn’t know the sub-group. He reported the outbreak to the World Health Organization in Geneva. WHO published the important news alongside reports of a similar outbreak in Hong Kong, where about 15%～20% of the population had become ill.
As soon as the London doctors received the package of throat samples, they began the standard tests. They found that by reproducing itself at very high speed, the virus had multiplied more than a million times within two days. Continuing their careful tests, the doctors checked the effect of drugs used against all the known sub-groups of type A virus. None of them gave any protection. This then, was something new; a new influenza virus against which the people of the world had no ready help whatsoever.
Having isolated the virus they were working with, the doctors now conducted tests on some specially selected animals, which contracted influenza in the same way as human beings did. In a short time the usual sign of the disease disappeared. Theses experiments revealed that the new virus spread easily, but that it was not a killer. Scientists, like the general public, called it simply "Asian" flu.

The influenza discovered by a doctor in Singapore is caused by ______.

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