The warning was given Monday by Dr. Margaret Chan, head of the 193-nation World Health Organization, in an interview with the Financial Times of London.
Chan warned that the swine flu virus known as H1N1 that caused the Mexico City-centered outbreak could return in the fall as a far more dangerous mutation.
After last week's warnings, school closings across the United States and the near shuttering of Mexico City, the current outbreak seems to have peaked. The WHO said Monday there were 985 confirmed cases of H1N1 spread over 20 countries. There have been 25 confirmed deaths.
As of Monday there were 286 reported cases of swine flu in 36 U.S. states. Both U.S. and Mexican authorities expressed confidence that the spread of the disease was slowing down.
The World Health Organization said the higher number of reports of cases from Mexico -- 590 -- comes from testing of previously gathered samples.
The four strands of the swine flu virus come from pigs, humans and birds. Experts believe that the virus mutated into its current form in the bodies of pigs. Health authorities are particularly worried that the capability to mutate already exhibited by the virus could eventually let it combine with the human immunodeficiency virus, which causes AIDS.
That could cause a lethally dangerous global health problem on a comparable scale to the 1918 Spanish influenza epidemic that eventually infected more than 500 million people -- more than one-quarter of the human race -- and killed 10 percent of them. That death toll of 50 million was more than five times the total fatalities of World War I. The epidemic killed more Americans than died in World War I and World II combined.
Canadian health officials said Sunday they have confirmed that the H1N1 swine flu virus had, in at least one case, leaped back into a herd of 200 pigs. That raised the possibility it could mutate again in pigs and move back into the human population.
Chan told the Financial Times that, given the potential scale of the possible threat, the World Health Organization did not overreact to the swine flu threat. While the number of new cases hasn't grown as fast as expected, Chan said the disease could return in a few months in a much more lethal strain. She also said she would rather be over-prepared than have to answer questions about why the World Health Organization didn't take sufficient action.
The reaction of the U.S. government headed by President Barack Obama and Homeland Security Secretary Janet Napolitano was measured, restrained and less tough than that of the 27-nation European Union or of nations like China in closing cross-border traffic or imposing comprehensive screening.
The Chinese government was horrified at the possibility that swine flu could spread among its 1.3 billion people, almost 20 percent of the human race. Its emergency measures, however, have infuriated the Mexican government and led to a major diplomatic row between the two nations.
Mexican travelers were quarantined in hotels, and the Mexican ambassador to China was not allowed to meet with one group he tried to visit. The anger of the Mexican government at the Chinese measures, however, has obscured the real possibility that the global impact of swine flu has been limited precisely because of the swift measures that were taken globally to contain it.
The global swine flu crisis recalls the so-called millennium bug, which was supposed to crash computers around the world as the machines' internal clocks turned over Jan. 1, 2000. That didn't happen, but some experts said that was because the precautions taken helped prevent the problem. Some said there wasn't a problem to begin with. The whole controversy revolved around a negative proposition that couldn't be proved.
Skeptics are already arguing that the global fever over swine flu should fall into the same category. However, human history is filled with little-known but horrifying examples of global pandemics from diseases like Spanish flu, cholera, syphilis or bubonic plague that swept the world, killing hundreds of millions of people, destroying civilizations and reshaping the demographic patterns of the planet.
In a modern world of unprecedented population scale and social mobility, Chan's caution therefore appears completely justified. The alternative is to risk a biological disaster that could eventually prove more devastating than a thermonuclear war.