Health experts walked a tightrope Sunday, unsure whether the swine flu epidemic was starting to fizzle out or was just in a lull before another surge, as Spain reported 20 confirmed cases, making it the hardest-hit nation in Europe.

Health Secretary Jose Angel Cordova said there were 11 cases of people suspected to have died in Mexico from the virus in the previous 24 hours. The alarming news came after the epidemic's toll in Mexico appeared to have been leveling off.

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.

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Swine flu cases have been confirmed in 19 countries so far — including in Europe, the Middle East and the Asia-Pacific region — and experts believe the actual spread is much wider.

On Sunday, Colombia diagnosed its first confirmed case of the H1N1 flu virus, the country's Social Protection Minister Diego Palacio said.

The Spanish Health Ministry said the country now has 20 confirmed cases of swine flu, making Spain the hardest-hit nation in Europe amid the swine flu outbreak. Five new patients were confirmed in the northeastern Catalan region, the ministry said. Another 99 people are still under investigation for the disease.

German health authorities said Sunday that the country now had eight confirmed cases. A married couple from the state of Brandenburg were just diagnosed — and they had been on the same flight as a man from Hamburg who had swine flu after visiting Mexico.

Italy reported its second case of swine flu Sunday, a 25-year-old man who recently returned from Mexico. The Health Ministry said the man is fine and was being kept in isolation at home.

Asia had no new cases Sunday, but Hong Kong and South Korea each reported one case on Saturday — the first on the continent — setting off alarms in a region with memories of severe acute respiratory syndrome and bird flu outbreaks. Both cases were in people who had recently arrived from Mexico.

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.

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."

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.

President Barack Obama 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 to share information.

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.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.

In Baghdad, Iraqi officials killed three wild boars at Baghdad's zoo because of swine flu fears, even though health experts say the virus is not transmitted by pigs. Iraq has no documented cases of swine flu.