It spread rapidly through international air travel to points around the world from what appear to be its origins in eastern Mexico.

That prompted the World Health Organization to raise the flu threat level to Phase 5, which the agency says indicates a pandemic is “imminent.”

A four-year-old boy in the Veracruz state village of La Gloria survived the first confirmed case of the disease in late March.

While it’s not believed the boy was directly infected by animals bred in a nearby U.S.-owned industrial pig farm, a strain of regular flu making others ill in the area may have mutated with swine flu and bird flu elements to form the new pandemic strain.

The H1N1 influenza strain is spread from person to person, instead of animal to animal, making it harder to treat and prevent since there is no natural immunity to it and no effective vaccine.

Health officials are puzzled by other cases of the strain that emerged about the same time in San Diego and the southern Mexican city of Oaxaca.

The Mexican government's chief epidemiologist, Miguel Angel Lezana, told correspondents on Wednesday that it’s possible the California victims could have been infected by a flu strain from Asia that traveled south, or that the flu was generated in Mexico and went north.

Photo: Samuel Allred