U.S. health officials warned of worsening outbreaks of the new H1N1 flu, and a top global health official predicted up to a third of the world's population could eventually become infected.

Asian countries, which have had few cases so far, pledged on Friday to increase stockpiles of flu medicine and cooperate in an emergency, taking no chances this time after the damage wrought by SARS and bird flu in recent years.

The CDC has reported 1,639 confirmed cases of the new H1N1 flu in 43 states. U.S. officials say they expect the virus to spread to all 50 states and cause many infections, ranging from mild to severe.

There have been two deaths in the United States. On Friday afternoon, officials confirmed the first Canadian death from the H1N1 flu virus.

"So far we are not seeing any signs of this petering out," said Dr. Richard Besser, acting director of the U.S. Centers for Disease Control and Prevention. "We are on the upswing."

A batch of detailed studies on the new "swine flu" virus showed it was a strange marriage between a triple-hybrid virus with pig, human and bird elements and a European swine virus not seen before in North America.

Mexico's confirmed death toll ticked up to 45 on Friday as labs tested a backlog of samples from people who died in March and April.

Health Secretary Jose Angel Cordova said tests also confirmed 159 more cases of swine flu, bringing the total number of people sickened to 1,319.

Though Mexican officials said they were over the worst, Besser said the country still had significant transmission.

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More than 2,000 people in 24 countries have been infected with the virus, the World Health Organization said.

The WHO's Dr. Keiji Fukuda urged Asian governments to stay alert for a possible wider pandemic that "could infect a third or more of the world's population in the next several months, in the next year.

"Even if the illnesses appear relatively mild on an individual level, with large numbers of infections on the global population, you can get large numbers of seriously ill people," Fukuda told health ministers from the 10-member Association of South East Asian Nations (ASEAN) plus China, Japan and South at a meeting in Bangkok on Thursday.

Fukuda said H1N1 flu is not yet spreading in a sustained way outside North America, so the global pandemic level remains at 5 out of 6.

According to a draft statement on Friday, the Asian health ministers were concerned that most of the production capacity for vaccines was located in North America and Europe and it was inadequate for a global pandemic.

The 13 countries will look at screening people leaving affected areas but are not planning travel bans.

Evidence showed that "imposing travel restrictions would have very little effect on stopping the virus from spreading, but would be highly disruptive to the global and regional communities and pose major negative impacts on the current global economic downturn," the statement said.

In a series of reports rushed into the New England Journal of Medicine on Thursday, researchers said influenza viruses from animals are an ongoing threat and doctors need to keep a lookout for them.

Mystifying Genes

So far the genetic analysis gives no clue about where the new virus came from, the researchers told a news conference. The European genes were especially mystifying.

"Those genes had never been seen in the United States before," the CDC's Dr. Michael Shaw said.

In Mexico, millions of high school and university students returned to classes as the country got back on its feet after shutting public places last week to avoid the spread of the disease.

But visitors to government-run buildings were asked to wear surgical masks and wash their hands with antibacterial soap before entering. Restaurants also sanitized diners' hands as they arrived.

The U.S. pork industry got some good news after being battered by import bans by nearly two-dozen countries.

U.S. Agriculture Secretary Tom Vilsack said the government "got this turned around."

Russia, the fourth-largest export market for U.S. pork, signaled it may lift bans placed on pork by June 1, a spokeswoman for the U.S. Trade Representative said.

The Associated Press and Reuters contributed to this report.