Mexicans put faith in masks, but do they work for flu

While Mexico has handed out millions of facial coverings, U.S. officials have held off, saying there is little evidence of their effectiveness. Some doctors warn they might even be harmful, causing people to take risks like venturing into crowds or neglecting to wash hands in the mistaken belief that the mask protects them.

The ubiquitous masks give an eerie, unsettling air to this overcrowded city, as if 20 million people have entered a scene from some kind of apocalyptic future. They're also a reminder of an equally frightening episode: Technicolor versions of those dotting scratchy black and white photographs from the 1918 Spanish Influenza epidemic, which claimed up to 50 million lives worldwide.

Soldiers hand them out at subway stations. Pharmacies and hardware stores can't keep them in stock. Newspapers have begun running front page instructions on making do-it-yourself mouth coverings. President Felipe Calderon proudly boasted over the weekend that more than 6 million masks have been distributed.

"They must be worn when one is out in public or in a closed, crowded space," Health Secretary Jose Angel Cordova insisted Monday, while acknowledging in the same breath that the government-distributed masks are too porous to eliminate all risk.