The virus spread to Colombia in the first confirmed case in South America, worrisome because flu season is about to begin in the Southern Hemisphere. More cases were confirmed in Europe and North America; health officials said at least 937 people have been sickened worldwide.
Dr. Richard Besser, acting chief of the U.S. Centers for Disease Control and Prevention, said H1N1 (swine) flu is spreading just as easily as regular winter flu.
"The good news is when we look at this virus right now, we're not seeing some of the things in the virus that have been associated in the past with more severe flu," Besser said. "That's encouraging, but it doesn't mean we're out of the woods yet."
"It's a rapidly evolving situation, and it's still one that is cloaked in uncertainty," Besser said on CBS' Face The Nation. "But each day we're getting more information … and we're starting to see encouraging signs."
On Sunday, health officials raised the number of confirmed U.S. swine flu cases to 244 in 34 states. The new number, up from 160 on Saturday, reflects streamlining in federal procedures and the results of tests by states, which have only recently begun confirming cases, said Dr. Anne Schuchat of the CDC.
Mexican Health Secretary Jose Angel Cordova said the virus that has killed 19 people in Mexico and sickened at least 506 apparently peaked here between April 23 and April 28. A drastic nationwide shutdown appears to have helped prevent the outbreak from becoming more serious, he said.
"The evolution of the epidemic is now in its declining phase," Cordova declared.
He said officials would decide Monday whether to extend the shutdown or allow schools and businesses to reopen on Wednesday.
Pablo Kuri, an epidemiologist advising Cordova, told The Associated Press on Sunday that tests have confirmed a swine flu death in Mexico City on April 11, two days earlier than what had been believed to be the first death.
He also said there have been no deaths among health care workers treating swine flu patients in Mexico, an indication that the virus may not be as contagious or virulent as initially feared.
The closed events made for a surreal Sunday in Mexico, as parishioners celebrated Mass via television, camera operators were the only ones in stadium bleachers and parks, museums, restaurants, theaters and other attractions were closed.
Sunday also marked the official start of campaigning for July 5 congressional elections - but all public campaigning was banned to prevent gatherings where the virus could spread.
Gabriela Cuevas Barron of the conservative National Action Party giddily claimed she was launching Mexico's first virtual campaign, promising in a Webcast to work for a cleaner and safer Mexico City for now, through Twitter, Facebook and YouTube. Meanwhile, the leftist Democratic Revolution Party was already up with a Facebook page for its candidates.
The United States said it had sent 100,000 protection kits worth $1 million to Mexico for use by first responders. The kits include respiratory masks, protective goggles and overalls. In all, the U.S. has sent $16 million in aid to Mexico since the emergency began, the U.S. Embassy said.
CBS News correspondent Bianca Solorzano reported that 470 schools have closed in at least 18 states, stranding some 270,000 children at home.
Officials in New Mexico announced Sunday that 14 schools in four towns were being closed for at least a week after the state's first swine flu case was confirmed, and the New Mexico Activities Association suspended all athletic and activity programs until further notice at member schools across the state.
In Arizona, all 10 public schools in the border city of Nogales canceled classes this week after a student tested positive for swine flu.
California officials, meanwhile, suspended all visitations at prisons pending results of tests on an ill inmate at Centinela State Prison.
In the Canadian province of Alberta, officials quarantined about 220 pigs that became infected from a worker who had recently returned from Mexico. It was the first documented case of the H1N1 virus being passed from a human to another species. Canada stressed that pigs often get the flu and there's no danger in eating pork.
Egypt has ordered all pigs in the country slaughtered as a precaution, sparking riots Sunday by pig farmers who threw stones at police.
According to tallies by the CDC, World Health Organization and governments, there were 101 confirmed cases of swine flu in Canada; 40 in Spain; 18 in Britain; eight in Germany; four in New Zealand; two each in Italy, France, Israel, and South Korea; one each in Colombia, Costa Rica, Ireland, Switzerland, Austria, Hong Kong, Denmark and the Netherlands.
Fear spread in other parts of the globe, much of it directed at Mexican nationals or those who have traveled to Mexico.
China quarantined more than 70 Mexican travelers in hospitals and hotels there, and Mexicans on arriving flights were being taken into isolation, said Mexico's ambassador, Jorge Guajardo. Even the Mexican consul in Guangzhou was briefly held after returning from a vacation in Cambodia, Guajardo said.
"In many cases we have gotten reports that they were being quarantined for the sole fact that they had a Mexican passport, whether or not they came from Mexico, whether or not they had been in Mexico, whether or not they had been in contact with someone else from Mexico," Guajardo said.
Hong Kong isolated 350 people in a hotel after a Mexican traveler there was determined to have the swine flu.
In Trinidad, crew aboard a Mexican tanker had been isolated since Friday at the Point Lisas Port. The Ministry of Health said Sunday they were tested and cleared of any flu infection and that the vessel was expected to be released.
Health officials around the world cautioned that despite encouraging signs, swine flu still poses a very real threat.
"Most experts would agree that the current outbreak that we are experiencing is mild to moderate in severity," Dr. Jon Andrus of the Pan American Health Organization said in a teleconference from Washington. "That is not to say that things cannot change very rapidly and very dramatically."
Under one scenario, the virus could peter out now, only to roar back in the fall when flu seasons begins. That's why health officials are watching the Southern Hemisphere so closely.
"Certainly, maybe, this current round of activity has peaked, but we are only 10 days into this outbreak," WHO spokesman Gregory Hartl told reporters in Switzerland. "I think we would want to wait a while before making a definitive decision."
2009 H1N1 Flu Outbreak Map:
This is a map depicting confirmed and suspected cases of the 2009 H1N1 outbreak, with contributors from all over the world, from a variety of backgrounds including health, journalism, technology.