There is a joke among flu researchers: “If you've seen one flu season, you've seen one flu season.” The joke is about the unpredictable nature of the flu virus．Every year it looks different, and every strain (类型) follows its own pattern — it's the reason why new strains like H1N1 are extremely difficult to predict.
Dr．Michael Osterholm is a former adviser to the U.S．Department of Health and Human Services．“I know less about influenza today than I did 10 years ago,” he says in a joking way．“Every stone we've turned over, we get more questions.”
The flu rectums every season and the world experiences terrible pandemics (全国或全世界范围流行的疾病), but researchers still do not understand why some strains infect people and others do not; they are not entirely sure about how the flu is transmitted; nor do they understand why some patients become seriously ill while others develop mild symptoms (症状)．As a result, when a new strain shows up — like H1N1 — they often have little information to fall back on, and the lessons of previous pandemics are only somewhat helpful．While researchers are still putting together a complete picture of H1N1, for example, its most striking difference with the seasonal flu is that the elder1y are not the most vulnerable (易受攻击的) population.
Influenza's unpredictable nature makes it a moving target for researchers, says researcher Allison Aiello at the University of Michigan．“Even if we had complete seasonal flu data from the past, it wouldn't be much helpful for a new strain of influenza,” she explains.
Whi1e researchers are frustrated by the holes in their knowledge, they say, however, that the pub1ic--health community is generally doing a very good job responding to H1N1 with seasonal flu data that do exist．Studying influenza, says Osterholm, is “like looking through the windows of a house you can't get into because the door is locked.” Gathering the data researchers do have is like “looking through the windows to get a pretty good picture of what the inside looks like.”
One thing researchers do know for sure: the best way for people to protect against H1N1 is to get the vaccine once it becomes available to them.
What do we learn about H1N1 from the passage?
A．In fact it is not a kind of influenza virus.
B．It is quite possible to predict it in theory.
C．Old people are more likely to contract it than kids.
D．Receiving vaccines will be effective to protect against it.
The underlined phrase “fall back on” in Para．3 probably means .
A．rely on B．pass on C．col1ect D．exchange
What do we know about previous seasonal flu data?
A．It is useless to study them.
B．It is still necessary to study them.
C．They are misleading most of the time.
D．They are much more helpful than expected.
Which of the following could be the best title for the passage?
A．Outbreaks of the flu B．Symptoms of the flu
C．Mysteries of the flu D．Risks of the flu