MEXICO CITY -- Mexico announced a return to "normalcy" on Monday, preparing to reopen businesses and schools even as the swine flu virus sickened more than 1,400 people in 20 countries.
World health officials said the global epidemic is still in its early stages and a pandemic could be declared in the days to come.
Mexican President Felipe Calderon said the crisis was waning at its epicenter, justifying Wednesday's end to a five-day nationwide shutdown he credits for reducing the spread of the virus.
Already, streets in the capital seemed more lively, with more vehicles and fewer people wearing face masks. Some cafes even reopened ahead of time. Calderon said universities and high schools will reopen on Thursday, and younger schoolchildren should report back to school on Monday.
"The school schedule will resume with the guarantee that our educational institutions are in adequate hygienic condition," promised Calderon, who called on parents to join educators in a "collective" cleansing and inspection of schools nationwide.
"This is about going back to normalcy but with everyone taking better care," Calderon said.
Parents and teachers will turn away children who are sick. The government is spending $15 million for detergent, bleach and soap to clean buildings, in a country where 12 percent of the 250,000 schools, about 30,000, lack running water or bathrooms.
Mexico canceled its biggest celebration of the Cinco de Mayo holiday, a re-enactment of the May 5, 1862, victory over French forces in the central state of Puebla. Other holiday events were canceled.
And experts inside Mexico's swine flu crisis center warned that the virus remains active throughout Mexico and could bounce back once millions return to work and school.
It also might get worse north of the border.
"The bottom line is that there hasn't been time for the severe illnesses to perhaps show up in the U.S. yet," said Marc-Alain Widdowson, a medical epidemiologist from the U.S. Centers for Disease Control and Prevention.
U.S. experts also urged caution, even as a New York City school reopened Monday after a spring break trip to Mexico led to as many as 1,000 people being sickened.
"We are by no means out of the woods," said Dr. Richard Besser, acting director of the CDC.
Health Secretary Jose Cordova insisted infections are trending downward after 27 deaths at the center of the epidemic, including a toddler from Mexico who died in Texas. He said those infected pass it on to an average 1.4 other people, near the normal flu rate of around 1.3. Cordova said soccer stadiums and concert halls could reopen if fans are kept 6 feet apart.
Other experts said the known cases are only a fraction of what's out there, meaning more illnesses could surface once crowds gather again in Mexico.
"It's clear that it's just about everywhere in Mexico. I think now there is considerable person-to-person transmission," Widdowson said. And now that the virus is taking off in the United States, chances of severe cases could rise as well.
"We've seen in many of the cases in Mexico, there's been sometimes five to seven days of being mildly ill with increasing respiratory distress and then being hospitalized, and then spending five days or a week in hospital, so that's a timeline of two weeks," he said.
As of Monday, Mexico had 802 confirmed cases, and U.S. cases grew to at least 383 in 36 states. Globally, the virus has reached more than 1,447 people in 20 countries, still in its early stages, according to the World Health Organization.
The WHO was studying whether to raise the pandemic alert to 6, its highest level, which would mean a global outbreak has begun. WHO uses the term pandemic to refer to geographic spread rather than severity. Pandemics aren't necessarily deadly.
"We do not know how long we will have until we move to Phase 6," WHO Director-General Margaret Chan said. "We are not there yet. The criteria will be met when we see in another region outside North America, showing very clear evidence of community-level transmission."
The Southern Hemisphere is particularly at risk. While Africa still hasn't reported any swine flu infections and New Zealand is the only country south of the equator with confirmed cases, winter is only weeks away.
Experts worry that typical winter flus could combine with swine flu, creating a new strain that is more contagious or dangerous.
Still, the U.N. health agency urged governments to avoid unproven actions to contain the disease, including group quarantines of travelers from Mexico and bans on pork imports.
"Let me make a strong plea to countries to refrain from introducing measures that are economically and socially disruptive, yet have no scientific justification and bring no clear public health benefit," Chan said in a video message to the U.N. General Assembly in New York.
Calderon said other governments have treated his citizens unfairly, and punishing nations that report outbreaks sets a bad precedent for future flu control efforts.
"If they weaken us economically or in other ways, Mexico will be able to focus much less attention and funds on this problem," Calderon said.
Mexico's Economy Secretary, Gerardo Ruiz, said Mexico will bring the issue to the World Trade Organization if other countries don't drop restrictive measures.
Beijing denied that it is discriminating by quarantining Mexicans and any other passengers who came in close contact with them, even those who don't show symptoms.
Among the passengers stuck in Chinese hotels was Briton Mark Moore, who urged China to lift the quarantine.
"The government is trying to show the world they are strong in organizing this," the 37-year-old director of a Singapore-based company said in a phone interview. "I need to be in Singapore now. I have loads of things to do."
A group of 25 Canadian university students and a professor also were quarantined in China over swine flu fears, said University of Montreal spokeswoman Sophie Langlois. The group does not have any symptoms, Langlois said.
China, Argentina and Cuba are among the nations banning regular flights to and from Mexico, marooning passengers at both ends.
Mexico and China both sent chartered flights to each other's countries to collect their citizens. Argentina also charted a flight to bring Argentines home.
And in a goodwill measure, China sent Mexico the final batch in a $5 million humanitarian assistance package consisting of masks, gloves, disinfectants, infrared thermal scanners and other items.
In Tokyo, 37 passengers and two flight attendants on a flight from Los Angeles were detained in a hotel after Japanese officials suspected one traveler of having swine flu.
They were released about 10 hours later when the passenger, a Japanese woman coming back from Las Vegas, tested negative for swine flu, American Airlines spokesman Tim Smith said.
The latest figures from Mexico suggest the virus may be less lethal and infectious than originally feared. Only 38 percent of suspected cases have turned out to be swine flu, and no new deaths have been reported since April 29. But Cordova acknowledged that about 100 early deaths in which swine flu was suspected may never be confirmed because mucous or tissue samples were not collected.
Widdowson said it's too early to say the outbreak is waning in Mexico, but the signs of progress are clear.
"What we have not seen in Mexico City is a huge, runaway epidemic, and I think that's totally clear. The hospital capacity has not been exceeded. So there hasn't been anything like the kind of picture that people might expect from a severe flu," he said.