(AP) -- As the number of swine flu cases in Mexico wanes and rises, experts are being forced to walk a public health tightrope - if they push their message too far and the virus fizzles out, they could lose credibility. But if they back off and it suddenly surges, they will be blamed.
Mexico reported three new deaths from the swine flu epidemic late Saturday from a virus that has killed 19 in people in Mexico, one in the U.S. and is spreading across Asia and Europe.
Health Secretary Jose Angel Cordova said there were 11 cases of people suspected to have died in Mexico from the virus in the last 24 hours. The alarming news came after the epidemic's toll in Mexico appeared to have been leveling off.
Just over a week into the outbreak, the virus largely remains an unpredictable mystery.
New developments also are raising more questions, including an announcement by Canadian officials Saturday documenting the first case of the H1N1 human virus jumping from a person to pigs on a farm. The infected farmworker had recently returned from Mexico and has since recovered. None of the pigs died.
Right now, one of the biggest hurdles is a lack of information from Mexico. A team of international and Mexican virus sleuths is trying to piece together an epidemiological picture of who's dying and where transmission began, while also uncovering just how it's attacking people with severe illness. But details are emerging slowly.
Late Saturday, Mexico's confirmed swine flu cases jumped by about 25 to 473, including the 19 deaths. A Mexican toddler also died in Texas days ago, for a worldwide total of 20.
Until more is known about the outbreak in Mexico, no one should be making any bold predictions, experts say.
"I think we need to hit the pause button, at least outside of Mexico," said Michael Osterholm, an infectious disease specialist at the University of Minnesota who has advised the U.S. government on flu preparations.
"This is a flu virus acting like a flu virus and causing, at worst, mild to moderate influenza," he said. "We have no room for complacency here, but we have to have a proportional response. What are the risks at the immediate time?"
U.S. President Barack Obama also urged caution Saturday.
"This is a new strain of the flu virus, and because we haven't developed an immunity to it, it has more potential to cause us harm," Obama said. Later, he spoke with Mexican President Felipe Calderon for about 20 minutes to share information.
The global caseload was nearing 800 and growing - the vast majority in Mexico, the U.S. and Canada. Costa Rica reported its first confirmed swine flu case - the first in Latin America outside Mexico. The only other fatality involved a Mexican toddler who died while in Texas.
Swine flu cases have been confirmed in 18 countries so far - including Europe, the Middle East and the Asia-Pacific region - and experts believe the actual spread is much wider.
Scientists warn that the virus could mutate into a much deadlier form.
"Influenza is unpredictable," said Dr. Tim Uyeki, an epidemiologist at the U.S. Centers for Disease Control and Prevention who has worked on SARS and H5N1 bird flu outbreaks. "There are so many unanswered questions. This is a brand new virus. There's so much we don't know about the human infectious with this virus."
Pablo Kuri, a Mexican epidemiologist, said three of the dead were children: a 9-year-old girl, a 12-year-old girl and a 13-year-old boy. Four were older than 60. The other nine were between 21 and 39 - unusual ages for people to die from flu because they tend to have stronger immune systems.
Although most of the dead were from Mexico City, they came from different neighborhoods in the metropolis of 20 million, and there were no similarities linking their medical backgrounds.
One theory for the deaths is that perhaps they sought treatment too late - falling sick an average of seven days before seeing a doctor. For those who recovered, the average wait was three days, said Hugo Lopez-Gatell Ramirez, deputy director of Mexico's Intelligence Unit for Health Emergencies.
Many of the sick around the world were people who had visited Mexico, including 13 of Britain's 15 cases.
The World Health Organization earlier announced that a pandemic was imminent, but it has decided against declaring a full pandemic alert. Still, that doesn't mean people can relax, said Dr. Mike Ryan, WHO's global alert and response director.
"These viruses mutate, these viruses change, these viruses can further reassort with other genetic material, with other viruses," he said. "So it would be imprudent at this point to take too much reassurance" from the small number of deaths.
In the Canadian province of Alberta, health and agriculture officials said about 220 pigs on a farm were quarantined after being infected by a worker who had recently returned from Mexico. They stressed that swine viruses are common in pigs, and there was no need for consumers to stop eating pork as long as it's handled properly and cooked thoroughly. The pigs are all recovering in the first documented case of the H1N1 human flu being passed to another species.
Associated Press writers Juan Carlos Llorca and David Koop in Mexico City contributed to this report.
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