Ominous similarities exist between the swine flu and the 1918 pandemic virus that killed millions around the globe, from a shared "death profile" of 20-to-40-year-olds to an ability to spread rapidly among humans, experts say.
Some warn the swine flu outbreak has all the hallmarks of a pandemic. Others say it may be no worse than normal flu.
The confirmed death toll in Mexico is 20, with a likely total of 149 deaths, according to a national health ministry spokesman. There are now 40 confirmed cases in the United States, all mild, in five states. Six "mild" cases have been reported in Canada but that number could rise as officials await test results from several provinces.
No one can predict whether the swine flu is capable of morphing into a strain capable of unleashing a severe global pandemic.
"The bottom line is, we really don't know," says Dr. Brian Ward, an expert on viruses, vaccines and pandemics at McGill University in Montreal.
Experts don't know because important numbers are still missing. No one knows how prevalent the illness is. There could be millions of infected people in Mexico, with only the most severe cases ending up in a clinic or hospital, meaning what we're seeing is only the severe end of the spectrum.
"If it's 80 deaths on a background of 2,000 cases, that's really, really bad," says Ward, associate director of The Research Institute of McGill University Health Centre. "If it's 80 deaths and 1,300 hospitalized on a background of 10 million, it's still perhaps worse than the seasonal flu and a funny pattern. But it's not the same kind of hair-raising scenarios that the former numbers would lead you to believe.