Scientists struggle to understand new swine flu virus

American health officials believe they are getting closer to answering those questions, or, at least, to ruling out wrong-headed theories."We've begun to knock off hypotheses," said Dr. Scott F. Dowell, director of global disease detection with the U.S. Centers for Disease Control and Prevention.

Among the factors disease detectives have discounted are Mexico's air pollution, secondary infections and poor health care. But they still do not know why so many Mexicans have died, although it could be because many more people actually have had the virus than health officials realize.

In Mexico, the virus is suspected of killing more than 150 people and sickening more than 2,400. Recent information suggests swine flu-related hospital admissions and deaths may have peaked and are declining, but no other country has shown any numbers close to those seen in Mexico.

The only other country to report a swine flu death is the United States, and that involved a toddler from Mexico who was visiting Texas with his family.The leading theory remains that the virus itself is not significantly different in Mexico, but that the outbreak has for some reason just hit harder there, infecting more people overall. The more people who are infected, the more likely there will be severe cases and even deaths.