"We do know that our veterinary colleagues at USDA (the U.S. Department of Agriculture) and elsewhere in the world are now looking to see if samples in freezers from pigs or other animals that might provide the missing link," Cox said.
"If we can determine the origin we can also take measures to ensure that the virus doesn't reemerge in a slightly different form," she said.
The researchers said they do not know how this particular virus acquired the ability to infect people. It does not have the usual mutations that allow animal viruses to jump into people and then to pass easily from one person to another.
Flu experts get worried when viruses go straight from animals to people. Usually they do not pass any further than one person -- for example the feared H5N1 avian influenza virus that has infected 429 people and killed 262 of them rarely passes from person to person.
But all three recent pandemics -- in 1918, 1957 and 1968 -- occurred when a new avian flu virus started infecting people.
So far sampling of the new H1N1 virus shows very little genetic mutation -- a sample from a patient in Mexico is virtually identical to samples from various U.S. states and other countries.
This "indicates that this virus may have been INTRODUCED INTO HUMANS IN A SORT OF SINGLE EVENT," Cox said. Or if more than one person was infected directly from an animal or other source, they were infected with genetically identical viruses.