A previously undetected strain of bird flu (H7N9) caught the world's attention this week after it infected and killed people in China。Scientists have been scrambling to understand how the virus emerged and, more importantly, whether it poses a public health threat or has the potential to cause a global pandemic。The good news is that, so far, there is no sign that the H7N9 virus is spreading from person to person。But experts say the H7N9 virus has mutated in a way that worries them a bit。

　　Here are some popular science questions and answers about the H7N9 bird flu virus summarized by the Associated Press。This knowledge helps explain what is known about the virus and why it matters。

　　Q: What virus is H7N9 and what do we know about it？

　　A: H7N9 is named for the way proteins bind on its surface。The virus has infected at least 11 people in China since February, killing four of them。Symptoms include respiratory problems such as fever and severe pneumonia。Much is still unknown about the virus, including how people get infected。But scientists say it contains genetic markers that can infect humans。The H7N9 virus is believed to be able to spread in poultry enclosures without causing illness in poultry。This allows the virus to spread undetected between groups, making it harder to track。Not only that, but now that poultry is surviving longer on farms, in markets or elsewhere, it may also produce more pollution。

　　Q: How concerned should the public be about the H7N9 virus？

　　A. For now, experts say, there is no reason to panic, but they are watching H7N9 closely。There is no evidence that the virus is spreading from person to person, and no cases have been reported outside China。Experts are collecting samples from patients, and the Chinese Center for Disease Control and Prevention is sharing the genetic sequence of the virus with foreign scientists。That would allow the world's top flu experts to study the virus, looking for variants that could make it more dangerous。Poultry and its products are safe to eat as long as they are cooked properly。

　　Q: Is there a vaccine？

　　A: There is no vaccine, but WHO and its partners are already isolating and identifying potential candidate viruses that could be used to make future vaccines if necessary。However, it will likely take several months to produce the first vaccines。

　　Q: How does the new virus change, and why is that important？

A: Although it is not yet clear, the virus appears to have mutated, making it more adaptable and able to grow at the normal body temperature of mammals。Scientists are working to determine which species could now host the new virus, and one possibility is pigs。Pigs are important because they share some basic biological similarities with humans and can act as "hybrid vectors" (multiple flu viruses form new strains) if infected with different types of bird flu at the same time.。Other possibilities are that the mutation occurred when poultry were infected, or after humans were infected by birds。"For me, the most important question is: what is the real reservoir of this virus?" said Dr. Webby, director of the WHO Influenza Center at St. Jude Children's Research Hospital in Memphis, Tennessee？It does seem to involve some mammalian host。”

　　Q: How is this bird flu virus different from the H5N1 virus that everyone was worried about in the past？

　　A: The H5N1 virus is very deadly in birds, which makes it easier to identify and eliminate infections in poultry。H5N1 is still a bird flu and has not taken root in other species, such as pigs。Most human infections are associated with contact with infected birds。Scientists have been closely monitoring H5N1 since it first appeared in Hong Kong in 1997。It reappeared in 2003 and devastated poultry in Asia, killing at least 371 people and nearly 60 percent of those infected。Experts have long worried that H5N1 could mutate into a form that spreads easily from person to person, triggering an outbreak of infectious disease。