A pandemic is an outbreak of illness that expands to different parts of the world.
Officials at the World Health Organization declared an A-H1N1 swine influenza pandemic as the number of infections has grown and spread across continents.
According to the WHO, 74 countries have reported cases of the swine flu virus, and 144 people have died, including more than 100 in Mexico.
WHO officials note that in most cases outside of Mexico, swine flu tends to cause very mild illness, but that it has the potential to mutate and become more severe.
A pandemic occurs when a new strain of virus infects people, who have little or no immunity to the illness, and it spreads easily from person to person.
U.S. health sources say one of the challenges of confronting a pandemic is developing and distributing medical supplies, including vaccines. There can also be economic repercussions if officials decide to enforce travel bans, and close businesses.
The world was hit by three influenza pandemics during the 20th century, including the Spanish flu in 1918-1919, which caused an estimated 40 million to 50 million deaths.
The deadly flu pandemic 90 years ago was fueled by a type-A human influenza virus of subtype H1N1, but it was very different from the current swine flu. The new swine flu A-H1N1 is a combination of fragments of pig, avian and human flu viruses, and the U.S. Centers for Disease Control says no vaccine has yet been developed that can prevent the outbreak from spreading.
U.S. and U.N. health sources say swine flu cannot be contracted through eating pork products. They say washing or sanitizing hands is the best method for preventing infection. Some experts say wearing a face mask in public may also help.
More information is available from the U.S. government at www.PandemicFlu.gov, and from the World Health Organization at www.who.int/csr/disease/swineflu/