JIM LEHRER: Swine flu continued its creep around the world. The number of cases went to over 1,000, spanning 20 countries, according to the World Health Organization.
In the United States, the Associated Press calculated nearly 300 cases in 36 states.
But overall, there was some indication the spread of the flu was on the wane.
NewsHour health correspondent Betty Ann Bowser has our lead story report.
BETTY ANN BOWSER, NewsHour correspondent: Even while some health officials were expressing hope that the worst is over, today a number of schools nationwide were not taking any chances. They closed their doors temporarily, hoping to halt the spread of the H1N1 virus.
Students here at the Excel Academy in Arvada, Colorado, just outside of Denver, are among the 300,000 nationwide who've been told to stay home. Officials made the decision to close the school last week after a student here tested positive for swine flu. The boy had been in Mexico last month and developed symptoms when he returned home.
But even as some schools close their doors, government officials said they were reconsidering the effectiveness of that strategy.
But there was a measured return to normalcy at the school in Queens, New York, today, where the largest U.S. outbreak -- 45 confirmed cases -- had been reported last week. Students at the St. Francis Preparatory Academy headed back to class after a five-day closure.
BROTHER LEONARD CONWAY, principal, St. Francis Preparatory School: The kids are excited. Everybody's feeling better. We've got to get back on track.
MATTHEW SCHULTZ, student: It's a little -- I'm a little worried about it. Like, everyone got sick and stuff, but it's not a big deal.
BETTY ANN BOWSER: New York City Mayor Michael Bloomberg was on hand to welcome the students back. He was cautiously optimistic.
MICHAEL BLOOMBERG, mayor of New York City: We can't guarantee that there will not be any more H1N1 cases, particularly in the school, but we think that very unlikely, and the important thing now is for the students and staff and for New Yorkers across the city to continue to be calm and confident.
BETTY ANN BOWSER: But confidence is something that seems to be waning with U.S. pork. Exports have dropped about 10 percent since the swine flu outbreak started, according to the U.S. Meat Export Federation.
Top officials at the United Nations and the World Health Organization said there are no imminent plans to raise its pandemic alert to its highest level, level six. Doing so would indicate an increase in the spread of the virus, but not necessarily an increase in the severity.
DR. MARGARET CHAN, director-general, World Health Organization: Phase six indicates that we are in a pandemic. 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.
BETTY ANN BOWSER: Around the world, fear continued to spread, as cases were confirmed in some other countries.
Colombia reported the first confirmed case in South America: a 42-year-old man who just returned from a trip to Mexico. Colombia's social protection minister said dozens more were suspected to be infected.
Throughout China, more than 70 Mexican travelers were being quarantined. And at a Hong Kong hotel, 240 guests were also confined. It's where one infected Mexican man stayed a single night. He's since recovered.
The Mexican government hopes to send charter flights to repatriate its citizens.
During a Sunday interview, Mexican President Felipe Calderon urged the world not to discriminate against his people.
FELIPE CALDERON, president of Mexico (through translator): It seems unfair to me that, after we were honest and transparent with the world, some countries or places are taking discriminatory measures because of ignorance and misinformation.
BETTY ANN BOWSER: While officials in Mexico City, the epicenter of the H1N1 outbreak, lowered the threat level, they announced that most businesses and public venues were expected to reopen after a five-day shutdown, but most schools will be kept closed until May 11th.
In Atlanta, Acting CDC Director Dr. Richard Besser said the signs in Mexico were promising.
DR. RICHARD BESSER, acting director, Centers for Disease Control: It appears that things are leveling off in Mexico. They are reporting less activity in Mexico City.
But as we're seeing here, they're seeing different things in different parts of the country. So some of the encouraging signs, the situation in Mexico is encouraging. Some of the initial lab studies are encouraging, the lack of some of the factors in the virus that have been associated with more severe disease in previous pandemics.
BETTY ANN BOWSER: The CDC said about one-third of confirmed U.S. cases were people who had traveled to Mexico and likely picked up the infection there.