"People are concerned about this situation," Besser said at a news conference. "We are worried as well. Our concern has grown since yesterday."
Sixty people in Mexico have died of the flu -- and so far, 16 of the deaths are confirmed cases of swine flu, news sources quote Mexican officials as saying.
World Health Organization spokesman Gregory Hartl told the Canadian news agency CBC that there have been some 800 cases in Mexico City, where schools are closed due to the outbreak.
Alarmingly, the flu outbreak in Mexico is striking healthy young people -- a pattern that would be expected if a flu virus new to humans emerged.
"Because these cases are not happening in the very old or the very young, which happens with seasonal influenza, this is an unusual event and a cause for heightened concern," Hartl said in a CBC interview.
That's not the only eyebrow-raising feature of the swine flu outbreaks. Infections have occurred in Mexico, California, and Texas -- where warm weather should mean the end of the normal flu season, says William Schaffner, MD, president-elect of the National Foundation for Infectious Diseases and chair of preventive medicine and infectious diseases at Vanderbilt University.
"Will we see this flu virus transmitted in the warm months? That would give us heartburn," Schaffner tells WebMD. "And is this a harbinger of things to come during our next flu season?"
Another disconcerting feature of the outbreak is that it's probably too late to contain it to limited geographical areas.
"We are seeing cases in Texas and California with no connection between them. This makes us think there has been transmission from person to person through many cycles," Besser said. "For containment we need limitation to a confined geographical area, and we have not seen that here."