MEXICO CITY -- Health officials say they're trying to unravel the riddle of why all but one of the 153 deaths in the swine flu outbreak occurred in Mexico.
Also puzzling investigators is why the disease has killed young adults, the demographic that should have the greatest resistance to the flu strain, CNN reported Thursday.
"They're good questions that we're asking, too," said Von Roebuck, spokesman for the U.S. Centers for Disease Control in Atlanta. "We're still young in this investigation and we're still trying to understand exposure in this country as well as exposure in Mexico."
Mexico reported 152 fatalities in flu-like cases since the outbreak began, seven confirmed as swine flu. Nineteen patients are confirmed as having swine flu. About 2,000 people have been hospitalized with symptoms.
A child from Mexico died of the disease in Texas.
A consensus may be emerging -- the disease in Mexico has been around longer than first thought and infected more people than investigators can confirm, CNN said.
The virus has genes from North American swine influenza, avian influenza, human influenza and a form of swine influenza typically found in Asia and Europe, Nancy Cox, CDC's Influenza Division chief, said.
Several factors could be behind the greater death toll in Mexico, Dr. Howard Markel, director of the Center for the History of Medicine at the University of Michigan, told CNN.
"They may have had cases for several months now and probably have a greater number of people who have the disease, probably tens of thousands," he said. "There may indeed be more cases in the United States. The snapshot we're seeing in the United States may be an incomplete snapshot."
The fatalities also may have had another co-factor, Markel said, such as taking other medicine or having a pre-existing condition or infection that would make them more vulnerable.
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