Wednesday, the World Health Organization raised its pandemic alert from four to five, the second-highest level. CNN.com reported that Dr. Margaret Chan, the U.N. agency's director-general, said the decision means that all countries should "immediately" plans.

Swine flu, a highly contagious respiratory disease, has a mortality rate of between 1% and 4%, according to the World Health Organization. While initially spread into humans by contact with pigs, the virus then mutates and spreads human to human.

As of 5 p.m. ET on Wednesday, the that the U.S. had 91 confirmed cases of swine flu, while there have been 367 confirmed cases in Mexico and nearly 500 across the world.