The story

The World Health Organization on Monday raised its pandemic alert level in response to the outbreak of swine flu that originated in Mexico.

The move from level three to level four on the WHO's six-level threat scale means the world body has determined the virus is capable of significant human-to-human transmission -- a major step toward a flu pandemic, said Dr. Keiji Fukuda, the agency's assistant director-general.

A pandemic is not considered "inevitable," Fukuda said. But U.S. Homeland Security Secretary Janet Napolitano said the move "does indicate that we have a serious outbreak of swine flu on our hands."

The World Health Organization confirmed 73 cases of swine flu Monday, but health officials in Scotland, California and Texas confirmed nine more, bringing the worldwide total to 82.

The total includes 40 cases in the United States, confirmed by the Centers for Disease Control. Read full article »

CNN's Sanjay Gupta, Ariel Crespo, Ted Rowlands, Al Goodman and Jeanne Meserve contributed to this report.

All About Mexico • Influenza • Centers for Disease Control and Prevention