STEVE INSKEEP, host:
Scientists are asking some fundamental questions about the swine flu virus. NPR's Joanne Silberner has this report on some of the things that are known and some that are not.
JOANNE SILBERNER: One of the reasons people have found this virus so alarming is because initially it seemed to be killing a lot of people in Mexico. Then U.S. data started coming in. The illnesses here have been relatively mild. Some people thought maybe there were two different viruses, but lab tests suggest it's the same one.
Anthony Fauci of the National Institute of Allergy and Infectious Diseases says there could be another explanation. Mexico may have had hundreds of thousands of infections, the U.S. far fewer, and the death rate may actually be the same.
Mr. ANTHONY FAUCI (National Institute of Allergy and Infectious Diseases): So it's still a very open question as to whether there is a significant difference, and if so, what is the reason for that difference?
SLBERNER: If there is a difference, it may be that the viruses actually are not identical, or there may be complicating factors in Mexico such as the presence of other microbes or poor nutrition or chronic lung problems. There is one thing that's known about the virus, how it came to be: a process called reassortment, where several viruses infect one animal and swap genetic parts. Arnold Monto is an epidemiologist at the University of Michigan.
Mr. ARNOLD MONTO (University of Michigan): Now we think this probably happens a lot, but the virus typically is not fit enough to transmit, and what's happened here is that the virus has changed in one way or another and is by chance able to transmit from human to human.
SILBERNER: And he says it's an especially usual reassortment.
Mr. MONTO: Because it's got pieces from a lot of different species and from different regions of the world.
SILBERNER: Genes from a bird, a human, and a pig, from Europe, Asia, and North America. Where the virus itself originated though is another mystery. The earliest known appearance was in Mexico, but it could have arrived from somewhere else.
The question a lot of people want the answer for is can the new virus be stopped? A vaccine would help. U.S. scientists have already started the month-long process of making one, says Bruce Gellin, director of the government's National Vaccine Program Office. The next step is to grow lots of it in the lab.
Mr. BRUCE GELLIN (National Vaccine Program Office): And how well it grows will be the most important factor of how much vaccine can be available in a given period of time.
SILBERNER: Flu manufacturers deal with this production issue every year when they make vaccines against whatever flu virus happens to be circulating at the moment.
Mr. GELLIN: Sometimes they grow well. Sometimes they don't grow so well, and that's still one of the things that's left to be seen.
SILBERNER: One among others about the virus. Will it fade away on its own as viruses sometimes do? And if it doesn't, will it be more like the 1918 pandemic flu which killed tens of millions of people worldwide? Or will it have the much lower death rate of a seasonal flu?
Joanne Silberner, NPR News. Transcript provided by NPR, Copyright NPR.
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