The Associated Press

"Our advice to governments is that they should check their level of preparedness for disease, and that includes for smallpox," said WHO spokesman for communicable diseases Iain Simpson.

WHO director-general Gro Harlem Brundtland has asked a team of experts to study whether the agency should change its current recommendation that governments should not inoculate against smallpox.

The vaccine can have potentially fatal side-effects in one case in 13,000. As the disease is no longer considered to be a threat to health, the WHO's advice has been not to vaccinate.

But the anthrax attacks in the United States, coming in the wake of last month's suicide plane hijackings, changed all that.

"We are concerned about smallpox just as we are concerned about a number of diseases," said Simpson.

Smallpox Is Contagious

Smallpox is caused by the variola major virus, and is marked by fever, headache and widespread blisters.

Unlike anthrax, it is highly contagious. However, vaccinations can be effective in treating the disease even after it has been contracted.

The WHO estimates that there are some 90 million doses of smallpox vaccine stockpiled by governments for civilian use, with a further unknown amount reserved for the military.

The U.S. government said last week that it had approached drug makers about the possibility of producing 300 million doses of smallpox vaccine.