Mexico City ? A unique strain of swine flu is the suspected killer of dozens of people in Mexico, where authorities closed schools, museums, libraries and theaters in the capital on Friday to try to contain an outbreak that has spurred concerns of a global flu epidemic.

The worrisome new virus — which combines genetic material from pigs, birds and humans in a way researchers have not seen before — also sickened at least eight people in Texas and California, though there have been no deaths in the U.S.

“We are very, very concerned,” World Health Organization spokesman Thomas Abraham said. “We have what appears to be a novel virus and it has spread from human to human.”

The outbreak caused alarm in Mexico, where more than 1,000 people have been sickened. Residents of the capital donned surgical masks and authorities ordered the most sweeping shutdown of public gathering places in a quarter century.

President Felipe Calderon said his government only learned late Thursday, with the help of international laboratories, what kind of virus Mexico is faced with. “We are doing everything necessary,” he said. “We understand the seriousness of the problem.”