The United States declared a national health emergency Sunday amid confusion about the severity of an outbreak of swine flu and the number of those affected. The U.S. government is set to ship roughly 12 million doses of flu medications from a federal stockpile if needed. 

Mexico is hardest hit and is contending with the worst of the flu outbreak which is suspected in the deaths of dozens of people and sickening perhaps 1,000. In the United States, at least eight cases have been confirmed with the infection. The causative virus is a genetic mix of human virus, bird virus from North America and pig viruses from North America, Europe and Asia.

“The outbreak of swine flu in North America is not a cause for alarm,” President Obama said Monday. "This is obviously a cause for concern and requires a heightened state of alert.”

As the United States launched border screening for swine flu exposure, Obama said the government is "closely monitoring" the emerging cases. 

Meanwhile, countries around the world were trying to prevent the outbreak from spreading. Increased border precautions and heightened scrutiny of airline passengers is taking place in Japan, Europe as well as China, Russia and Taiwan.

Homeland Security Secretary Janet Napolitano altered plans to help prepare and respond to the swine flu outbreak. Twenty cases of swine flu have been confirmed so far in the United States, in New York, Kansas, California, Texas and Ohio. Many of them had recently returned from Mexico. 

All of those sickened in the United States have recovered or are recovering. Health officials cannot yet explain why the outbreak in Mexico has caused deaths and elsewhere has not.