LONDON " For years, scientists have warned a global flu epidemic is imminent, but the world's attention focused on bird flu " not swine flu.
The 2004 outbreak of bird flu that wreaked havoc in Asia and beyond led governments and international health organizations to marshal enormous resources into fighting the dreaded H5N1 virus.
Did the focus on bird flu distract scientists from other viral threats, opening the way for swine flu to catch the world by surprise?
Most experts say no " though they admit swine flu could have been watched more carefully.
"The H5 activity was warranted," said Richard Webby, a flu expert at St. Jude Children's Research Hospital in Tennessee. "Swine flu could sure have done with some of the attention, but it wasn't completely ignored."
Health officials dismiss any notion that they may have missed warning signs that could have alerted them to the crisis that erupted this month in Mexico, killing more than 150 people, and quickly spreading across borders to Europe, the Middle East, North America and Asia.
Scientists also have been caught off guard by the fact that the new swine flu is a variant of an old virus that has been around in humans for decades " leading many experts to rule it out as a possible pandemic source.
Global flu outbreaks come from entirely new forms, like the H5N1 bird flu, that don't easily infect humans.
Joseph Domenech, chief veterinarian at the Food and Agriculture Organization in Rome, said the new swine flu may have mutated into its current deadly form at the human-to-human level " meaning monitoring unusual flu cases in humans, rather than pigs, might have been a more important factor in spotting the new virus.
"The (genetic) reassortment may have occurred in humans, not animals," he said. That may explain why so few cases of human flu appear to have any link to pigs.
As health officials believe the new virus already is jumping with reasonable ease between humans they have not even recommended slaughtering pigs " the measure taken with poultry to stop avian flu's spread.
Elizabeth Mumford, an animal flu expert at the World Health Organization, said researchers have been aware of the possible dangers of swine flus because pigs have receptors in their respiratory systems that can pick up both bird and mammalian flus.
Flus often kill birds, preventing the virus' spread. But pigs are more hardy. They can mix and match different flus inside their bodies, which might then infect other animals, and in rare instances, humans.
But she said tracking swine flus is tricky: many cause only mild symptoms, without farmers ever noticing their pigs were sick " and no public health authorities ever being tipped off. "We as a global community know very little about the prevalence of swine flu," said Mumford.
And while bird flu comes in deadly and less deadly versions that give scientists clues into which ones might be more dangerous for humans, there isn't such a helpful distinction for swine flu, she said.