A strain of swine flu found in eight people in California and Texas has been linked to the deadly flu that has killed as many as 60 people and left 900 sick in Mexico.
On Friday, Mexico's federal government closed museums, libraries, state-run theaters and schools in its overcrowded capital in order to prevent the pig flu from spreading.
World health officials worry a global flu epidemic could spread from the city of 20 million.
We are very, very concerned, WHO spokesman Thomas Abraham. We have what appears to be a novel virus and it has spread from human to human.
It's all hands on deck at the moment, he said.
The Mexican Health Secretary, Jose Angel Cordova, told TV viewers: “The department of health advises everyone to avoid busy places and crowded events unless it’s absolutely necessary.”
Cordova said that virus that mutated from pigs and then at some point was transmitted to humans.
Mexican children and the elderly are being vaccinated to boost the immunity of those considered the most vulnerable in the community.
The U.S. Centers for Disease Control and Prevention say tests confirm that some of the Mexican victims died from the same new strain of swine flu that sickened eight people in Texas and California. It's a frightening new strain that combines genetic material from pigs, birds and humans, the organization said.
President Obama had been informed and is following the outbreak closely, an administrative official said Friday.