From the Associated Press:

WHO says the phase 4 alert means sustained human to human transmission causing outbreaks in at least one country. It signals a significant increase in the risk of a global epidemic, but doesn’t mean a pandemic is inevitable.

Many experts think it may be impossible to contain a flu virus already spreading in several countries.

WHO has confirmed human cases of swine flu in Mexico, the United States, Canada and Spain. Only Mexico has reported deaths from the new strain.