A timeline of events in the swine flu outbreak:

— December 2005 to January 2009: The Centers for Disease Control and Prevention receives reports of 12 cases of human infection with swine flu. Five of these 12 cases occurred in patients who had direct exposure to pigs and six reported being near pigs. Exposure in one case is unknown.

— March 28: Believed to be the date of the earliest onset of the swine flu cases in the U.S., Dr. Nancy Cox of the CDC said in an April 23 press briefing.

— April 2: A 4-year-old boy contracted the virus before this date in Veracruz state, Mexican Health Secretary Jose Angel Cordova later said citing test results. A community in Veracruz has been protesting pollution from a large pig farm.

— April 6: Local health officials declare a health alert due to a respiratory disease outbreak in the Mexican town of La Gloria in Veracruz state. Health officials record 400 cases of people who sought medical treatment in the previous week in the town. About 60 percent of the town of 3,000 are affected.

— April 17: CDC determines that two children in adjacent counties in southern California had illnesses caused by infection with swine flu. Both children became sick in late March.

— April 22: CDC confirms three additional cases of swine flu in California and two in Texas, near San Antonio.

— April 22: The Oaxaca Health Department indicates that 16 employees at the Hospital Civil Aurelio Valdivieso have contracted respiratory disease.

— April 24: Mexico's Minister of Health confirms 20 deaths from swine flu, but 40 other fatalities were being probed and at least 943 nationwide were sick from the suspected flu. Mexico City shuts down schools, museums, libraries, and state-run theaters across the capital.

— April 26: The number of confirmed cases in the U.S. climbs to 20 in five states. Mexico reports suspect clinical cases have been reported in 19 of the country's 32 states. Canada confirms six cases.

— April 27: The World Health Organization raises its pandemic alert status to Phase 4, meaning there is sustained human-to-human transmission of the virus causing outbreaks in at least one country.

— April 28: Cuba suspends flights to and from Mexico, becoming the first country to impose a travel ban.