So far, U.S. cases of swine flu have been milder than those seen in Mexico, where the World Health Organization has confirmed that at least 20 people have died from swine flu; health officials are investigating dozens more deaths in Mexico.

CDC officials yesterday said they expected more swine flu cases to be reported as public health officials heighten their hunt for the new strain of swine flu virus, which is a mix of swine, human, and bird flu viruses.

Countries around the world are watching for the virus, and scientists are scrambling to learn more about the virus and stop it before it becomes a pandemic.
An influenza pandemic occurs when a new influenza type A virus emerges for which there is little or no immunity in the human population, begins to cause serious illness, and then spreads easily from person to person worldwide, according to background information from the U.S. Department of Health and Human Services.