"We are closely monitoring the emerging cases of swine flu in the United States," he said. "And this is obviously a cause for concern and requires a heightened state of alert. But it is not a cause for alarm."
The virus is no longer just a North American problem. With Europe's first confirmed case of swine flu in Spain, the European Union's health commissioner has warned Europeans to avoid nonessential travel to Mexico and the United States. There are also suspected or confirmed cases in the United Kingdom, France, Israel and New Zealand; six confirmed cases in Canada – all among those who recently traveled to Mexico.
Meanwhile, in the United States, the CDC has been trying to quell American's fears about the disease that may have already claimed the lives of 149 people in Mexico, according to the country's health minister. Nearly 2,000 people have been hospitalized since mid-April and only half have been released. Some say the danger of a pandemic is real.
Mexico's first suspected case of the swine flu was detected in the southern state of Oaxaca, Health Minister Jose Angel Cordova said Monday, according to Reuters. He said it was too early to identify the cause or geographical source of the disease.
"It's a new virus, new virus combination, it does transmit from person to person and we already know it causes fatalities so we already have all the makings of a possible pandemic," Irwin Redlener of Columbia University's Mailman School of Public Health said of a potential severe outbreak that spreads to a wide geographic area.
The CDC's Besser said that although the swine flu is highly contagious, there are simple precautions Americans can take to stop the spread of the virus: washing their hands, staying away from crowded places and staying home if they feel ill.
Because there have not been any deaths in the United States, so far authorities have said the virus may have mutated into a weaker form after first spreading from pigs to humans. Besser said officials in the United States are trying to get more information from Mexican authorities to determine why their cases have been more severe.
Hayden Henshaw's family has been quarantined in their Cibolo, Texas, home after the teenager caught the swine flu. His father and sister later came down with the virus, showing how fast it can spread.
"My muscles hurt, my skin hurt, I couldn't pick my head up out of bed," Henshaw said.
Robin Henshaw, Hayden's mother, is the only member of the family not to come down with the swine flu.
"I'm just washing my hands a lot," she said. "We're using a lot of Lysol and we're taking Tamiflu."
While Mexico City -- considered to be this outbreak's ground zero -- has been largely deserted after an order came to effectively shut the city down for the next 10 days, the United States is taking swift action to ensure the flu doesn't cause similar problems here.
Border crossings and airports are the frontlines of the battle to halt the spread of the disease. More than 13 million people traveled from Mexico to the United States last year, according to the Department of Commerce, and 20 million Americans traveled to Mexico last year alone.
At the San Ysidro border crossing, where about 80,00 people cross daily, custom agents are on the lookout for those who may be sick -- taking those in question to a secondary holding facility for evaluation.