(AP) -- Why has the swine flu engulfing Mexico been deadly there, but not in the United States?
Nearly all those who died in Mexico were between 20 and 40 years old, and they died of severe pneumonia from a flu-like illness believed caused by a unique swine flu virus.
The 11 U.S. victims cover a wider age range, as young as 9 to over 50. All those people either recovered or are recovering; at least two were hospitalized.
"So far we have been quite fortunate," said Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention on Saturday, just hours before three new U.S. cases were confirmed.
Health experts worry about a flu that kills healthy young adults - a hallmark of the worst global flu epidemics. Deaths from most ordinary flu outbreaks occur among the very young and very old.
Why the two countries are experiencing the illness differently is puzzling public health experts, who say they frankly just don't know.
It may be that the bug only seems more deadly in Mexico.
And while experts believe Mexico is the epicenter of the outbreak, they're not certain if new cases are occurring or if the situation is getting worse. They also don't know if another virus might be circulating in Mexico that could be compounding the problem.
A big question is, Just how deadly is the virus in Mexico?
The seasonal flu tends to kill just a fraction of 1 percent of those infected.
In Mexico, about 70 deaths out of roughly 1,000 cases represents a fatality rate of about 7 percent. The Spanish flu pandemic of 1918-19, which killed an estimated 40 million worldwide, had a fatality rate of about 2.5 percent.
The Mexican rate sounds terrifying. But it's possible that far more than 1,000 people have been infected with the virus and that many had few if any symptoms, said Dr. Michael Osterholm, a prominent pandemic expert at the University of Minnesota.
U.S. health officials echoed him.
"In Mexico, they were looking for severe diseases and they found some. They may not have been looking as widely for the milder cases," said Schuchat of the CDC.
The U.S. health agency sent two investigators to Mexico on Saturday to help, she said.
Currently, even the counted illnesses are problematic. Only a fraction have been lab confirmed. Severe penumonia-like illness happens all the time, so it's challenging to figure out which ones are really tied to the outbreak, U.S. health officials said.
"These numbers need to be confirmed," said Dr. Richard Wenzel, the immediate past president of the International Society for Infectious Diseases.
Other ideas about the difference include:
-Genetic analysis of virus samples in the two countries is continuing. The CDC says tests results show the U.S. and Mexican viruses are essentially the same, but some experts have not ruled out the possibility that the virus is changing as it leaks across the border to the north.
-Perhaps nutrition levels are worse in some Mexican communities - poor nutrition can degrade a person's immune defenses, and make them more susceptible to illness.
-Air quality in Mexico City is considered terrible. That too may have affect patients confronted with a novel respiratory disease.
-Access to medical care has been an issue in Asia, where a rare bird flu - which does not spread easily from person-to-person - has killed more than 200 over the last several years. Maybe Mexican patients have also had trouble getting medical care or antiviral drugs, some have speculated - even though the government provides health care.
All that is speculation at this point.
"The question of why the virus appears to be more virulent in Mexico is one that we are looking intensively into," the CDC's Schuchat said. "Rather than speculate, it's important for the science to lead us on this."
• Join PhysOrg.com on Facebook!
• Follow PhysOrg.com on Twitter!
Associated Press Writer Mark Stevenson in Mexico City contributed to this report.
©2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.