Wash your hands.

MEXICO CITY – Mexico City closed schools across the metropolis of 20 million Friday after at least 16 people died and more than 900 others fall ill from what health officials suspect is a new strain of swine flu. World health officials worried that it could mark the start of a flu pandemic.

The World Health Organization in Geneva, Switzerland said at least 57 have died in the outbreak, although it wasn't yet clear if this larger number of deaths was due to swine flu.

"We are very, very concerned," said Thomas Abraham, a spokesman for the agency. "We have what appears to be a novel virus and it has spread from human to human." If international spread is confirmed, that meets WHO's criteria for raising the pandemic alert level, he added.

Abraham said WHO on Friday raised their internal alert system, allowing them to divert more money and personnel to dealing with the outbreak. "It's all hands on deck at the moment." Abraham said.

Mexico's Health Secretary, Jose Cordova, said only 16 of the deaths have been confirmed to have been caused by the new strain, through testing at the government's laboratories. Samples from 44 other people who died were still being tested. The health department put the total number of people sickened at around 943 nationwide.

Cordova said samples were sent to the U.S. Centers for Disease Control and Prevention in Atlanta, Georgia, to determine whether it's the same virus infecting seven people in Texas and California. As of now, tests show the flu is a "new, different strain ... that originally came from pigs."

"We certainly have 60 deaths that we can't be sure are from the same virus, but it is probable," Cordova told MVS radio in Mexico City.

Cordova described a chilling new strain that had killed only people among the normally less-vulnerable young and mid-adult age range. One possibility is that the most vulnerable segments of the population — infants and the aged — had been vaccinated against other strains, and that those vaccines may be providing some protection.

But Dr. Anne Schuchat of the CDC said "at this point, we do not have any confirmations of swine influenza in Mexico" of the kind that sickened seven California and Texas residents.

All seven U.S. victims recovered from a strain of the flu that combines pig, bird and human viruses in a way that researchers have not seen before.