Officials advised Monday against most travel to Mexico, the center of an outbreak of swine flu suspected of killing almost 150 people there and sickening at least 50 through its spread to the United States.
The acting director of the Centers for Disease Control and Prevention said cases of the virus in the U.S. have been mild - none has been reported in Maryland - but warned that more serious cases could emerge.
He said officials were reacting "aggressively," including releasing 11 million courses of anti-viral drugs from a national strategic stockpile and sending kits to some states to enable them to test for the disease locally.
On Monday, a day after federal authorities announced a public health emergency, President Barack Obama told a group of scientists that while the outbreak is a cause for concern, it is "not a cause for alarm."
Later in the day, the State Department issued an alert advising U.S. citizens to avoid nonessential travel to Mexico.
Maryland health officials said they are working with hospitals and health departments, bracing for what they predict will be the inevitable stricken patient.
"We will have a case here in Maryland," said state health Secretary John M. Colmers. "I don't think there's any doubt of that. What we don't know is how extensive it will be and whether or not it will be as virulent as what we are seeing in Mexico. That's why we must continue to monitor the situation."
Worries about the outbreak set doctors' phone lines ablaze, led to reports of runs on surgical masks in some cities and roiled the economy, sending stocks lower on fears that the tourism industry could be further hobbled by restrictions brought on by swine flu.
Globally, the World Health Organization raised its alert level but stopped short of calling the outbreak a pandemic.
In Mexico, the illness has infected about 2,000 people and is suspected of claiming about 150 lives, although not all deaths have been confirmed as resulting from flu. It has shut schools, closed churches and emptied streets in Mexico City. One case has been confirmed in Spain and one is suspected in France, prompting European officials to warn citizens against visiting the U.S. and Mexico.
The CDC's Besser called that warning, in the case of the U.S., "quite premature."
Domestic cases have been reported in New York, Texas, California, Ohio, New Jersey and Kansas, according to the CDC. As of Monday, a total of 28 confirmed cases were from one New York City school.
Officials know that this flu appears to be spreading from person to person, but they are not sure how virulent it is. Younger people in Mexico have died from it, but not the babies and older people who are most endangered by seasonal flu.
This year's flu shots are ineffective against the strain, but CDC scientists are considering whether to develop a vaccine that includes the strain for the fall, a difficult undertaking.
"Over the course of the next week or two, we'll know a whole lot more," Besser said.
The new virus is part human, part avian and part porcine. "It has a mix of new genes," said Dr. Ruth Karron, director of the Center for Immunization Research and the Johns Hopkins Vaccine Initiative at the Johns Hopkins Bloomberg School of Public Health. "It's a virus that humans have not previously been exposed to, so there's no immunity to this flu. We're broadly susceptible to this flu."
Flu experts have been on high alert in recent years, trying to prepare for the next unanticipated strain.
"We were all thinking about bird flu," Karron said. "We were looking to Asia for the development of the next pandemic strain, and here we have a swine flu outbreak in the Americas. It's not where we were expecting to see it."