"The genetic make up of this virus has influenza experts scratching their heads," he said. "One of the things that has us worried is that could this be a virus that could continue to make mischief during the warmest parts of the year. That would be a big thing. For a respiratory virus to be active during the summer months" would be very unique.
The escalation in the swine flu situation on Saturday underscored concerns by international, federal and local health agencies over the threat of the new virus, even as government health officials said much remains unknown.
But if one thing is clear about the spread of this virus, it is that containment is no longer an option.
Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases said broad spectrum of the illness is expected in the United States.
"It's clear that this is widespread," said Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, at a press conference Saturday afternoon.
"We do not think that we can contain the spread of this virus," Schuchat added. "Having found virus where we have found it, we are very likely to find it in other places. ... We are not at a point where we can keep this virus in just one place."
The cases further demonstrate that health officials do not know where else the virus might turn up. Details of swine flu's spread in Mexico also remain murky. A team sent by the Centers for Disease Control and Prevention is now on the ground there seeking clues on the origin and spread of the disease.
"What we still don't know is how widespread it is," said Schaffner. "The question is: How long have things been going on in Mexico, and how attentive have they been in terms of what's going on in their country?"
Schaffner added that had it not been for the death cases in Mexico, the swine flu would have largely gone unnoticed in the United States.
"My observation has been, were it not for the problem in Mexico, this would have been on page 15 instead of page 1," he said. "Because each year we have dominant influenza strains but we also have occasional strains that don't fit the dominant patterns."
Meanwhile, emergency departments in affected areas are preparing for what they said could be a rise in cases.
"It is quite conceivable that it could escalate fairly rapidly," said Dr. Richard Bradley, chief of the Division of EMS and Disaster Medicine at the University of Texas Health Sciences Center at Houston. "If we are seeing a flu epidemic in Mexico, we should expect an uptick fairly quickly in Texas, where I am, and in California."
Still, Bradley said that the situation thus far has been relatively fortunate.
"Right now, we are in the increased awareness phase -- we don't yet know how this is going to develop," he said. "Fortunately, right now the cases in California and in Texas seem to be milder than the cases we've seen in Mexico. We hope that this trend continues in the U.S."
Emergency departments in other areas of the country also are stepping up efforts to control the virus should it surface.