America's front line of defense against biological warfare has girded for battle — but the armor comes in the form of smallpox vaccinations from the Centers for Disease Control and Prevention.

The country's nerve center for disease research has vaccinated some of its health workers against the deadly, extremely contagious smallpox virus. These special-operations forces of the medical world would be ready in case they need to investigate a terrorist attack involving the virus, a spokesman said.

"We are putting together several teams that could be quickly dispatched to the field if we did see a suspected case of smallpox," CDC spokesman Tom Skinner said Sunday.

The CDC is taking the precaution even though there's no evidence that anyone is readying a terrorist attack using smallpox, which was eradicated outside laboratories 21 years ago. But with the anthrax attacks and fear of more terrorism to come, officials of the federal agency say it is important they be prepared — especially against one of the most pernicious plagues to afflict mankind.

But, just as with anthrax, the agency is expecting a number of false alarms, CDC director Dr. Jeffrey Koplan said.

In addition to vaccinating several dozen people on its crack epidemiological teams, the CDC will this week begin a series of training courses on smallpox for some employees and state and local health workers.

The smallpox virus is known to survive only in laboratories in the United States and Russia. However, germ warfare experts suspect that other countries, including North Korea and Iraq, may have secretly obtained stocks.

The virus can spread quickly from person to person and has a high death rate. The infection is characterized by a rash and a fever of at least 102 degrees.

The supposedly successful war against smallpox may have actually made American more vulnerable to the disease. Many Americans were never vaccinated, or were vaccinated but have decreased protection because the vaccine has worn off. The United States stopped smallpox immunizations in 1972.

Skinner said the CDC is not calling for public vaccinations now.

Since smallpox was eradicated, the CDC has sent epidemiologists to investigate suspect illnesses a few times a year.

Smallpox experts were sent to evaluate specific cases three times last month, said Dr. James Hughes, who directs the agency's center for infectious diseases. None of the patients had smallpox.

The Associated Press contributed to this report.