Interactive: Swine flu’s spread, April 17 -- May 6

The first known case of this never-before-seen strain of flu occurred April 1 in a 4-year-old boy who lived near a pig farm in Perote, a mountainous town east of Mexico City. A week and a half later, a 39-year-old woman died in Oaxaca, about 175 miles to the south. On April 23, a Canadian lab confirmed she had the new strain of the flu. By then, the H1N1 virus was in the U.S.