MEXICO CITY, Mexico (CNN) -- Mexico lowered its health alert level Monday, citing improvements in the battle against swine flu. The level was lowered from red, or "high," to orange, or "elevated." "The measures we have taken, and above all the public's reaction, have led to an improvement," Mexico City Mayor Marcelo Ebrard said at a news conference. "But I insist that the virus is still present, that we need to remain on alert, and the resumption of activities will be little by little, not all at once." But U.S. health officials cautioned that the H1N1 virus is still on an "upswing" in the United States. And the World Health Organization warned that after the number of cases begins to subside, the swine flu could return this year "with a vengeance." The number of confirmed cases worldwide was expected to cross the 1,025 mark Monday. Earlier in the day, the WHO reported at least 985 cases across 20 countries. There were 26 reported deaths, including one in the United States, a Mexican toddler who was visiting relatives in Texas. The U.S. Centers for Disease Control and Prevention reported 226 cases across 30 states. New York Mayor Michael Bloomberg announced Monday that 10 more cases not counted in the CDC total have been confirmed, bringing New York City's total to 73. One school -- P.S. 177 in Queens -- remained closed Monday because of five confirmed cases there. But St. Francis Prep School in Queens re-opened its doors. "The school has been completely sanitized," St. Francis Principal Leonard Conway said in a letter to students and their parents. More than 100 students were out sick two weeks ago; some were found to have the swine flu, becoming the first confirmed U.S. cases. The school shut down. Most of the students had mild symptoms, and all recovered, health officials said. Students had mixed feelings about returning. Some said they weren't afraid, but one said, "I'm a little nervous." "We're glad to see each other again," one boy added. Bloomberg visited to the school to welcome them back. In Mexico, where the worldwide outbreak began, officials said the wide-ranging closure of public venues might be lifted by Wednesday.