Officials in Mexico City have shut down schools, museums and other public venues because of a swine flu outbreak that has killed at least 20 people and may be responsible for 40 other deaths.
Many people in the Mexican capital were wearing face masks while in public. Authorities say 1,000 people have become ill.
Tests show that some of the Mexico victims died from the same new strain of virus that sickened eight people in California and Texas. But authorities say the U.S. cases have been mild and that all eight people recovered. They say none of the U.S. patients had any contact with pigs.
The acting director of the U.S. Centers for Disease and Control Prevention, Richard Besser, said Friday that health officials do not know if the outbreak will become a pandemic.
The World Health Organization said it was convening an expert panel to consider raising the pandemic alert level.
Health officials say the unusual strain of flu contains genetic material from pigs, birds and humans. But authorities have not called on Americans to avoid travel to Mexico.
The CDC says swine flu usually occurs in people who have been exposed to pigs, although human-to-human transmission is also possible. Symptoms resemble the regular human seasonal influenza and include fever, lethargy, lack of appetite and coughing. Its Web site says some people with swine flu experience a runny nose, sore throat, nausea, vomiting and diarrhea.
The CDC also says that between December 2005 and February of this year, 12 cases of human swine flu infection were reported.
U.S. officials say the White House is monitoring the situation.