The World Health Organization (WHO) has confirmed swine flu cases in two more countries—the U.K. and New Zealand—bringing the total number of affected countries to at least six. Although the disease appears to be causing only mild illness in most areas (Mexico being the notable exception), experts warn the virus could still cause a catastrophic global pandemic.

Last night, there were 73 laboratory-confirmed cases of swine flu (40 in the U.S., 26 in Mexico, six in Canada, and one in Spain); the tally has since increased to 79, with one new case in Spain, two in the U.K., and three in New Zealand, Keiji Fukuda, the WHO's assistant director-general for health security and environment, told reporters this morning. Meanwhile, media outlets are reporting that Mexico may have as many as 2,000 suspected cases and 150 deaths.  

"We still do not have a good explanation for why the pattern of cases in other countries seems to be mild" while there have been lethal cases in Mexico, Fukuda said, noting that scientists are still trying to figure out if there is an especially deadly form of the virus circulating in Mexico (unlikely, he says), or if there is something about the Mexican victims that makes them more vulnerable.

According to Jeffrey Kahn, a pediatrician and infectious disease expert at Yale University School of Medicine in New Haven, Conn., it could be that more people are dying in Mexico because they are getting secondary bacterial infections (which occur when bacteria invade lung tissue ravaged by a viral infection). Some studies suggest that bacterial pneumonia actually killed most victims of the 1918 Spanish Flu pandemic, which claimed as many as 50 million lives worldwide—more than World War I. Kahn also notes that populations with inadequate nutrition or limited prior exposure to flu viruses may also be more prone to severe swine flu infections.

But just because the virus has so far caused only moderate illness in most of the world, that does not mean it won't reappear with a vengeance in the future, Fukuda warns, pointing to the 1918 Spanish Flu, which began as a mild epidemic.

"If this virus behaves like a typical influenza virus, there is a seasonality to [its] distribution," Kahn says. "It might disappear in the summer and reappear with the other flu viruses in the fall and winter," he says, noting that it could mutate in the interim to take on more virulence.

"We have to be very, very cautious that if the epidemic sort of peters out over the next couple of weeks, it does not mean it's the end of the story," Kahn says. "It could be just the beginning."

See our in-depth report for more on the swine flu outbreak.