The Pentagon's plan to inoculate troops in the Persian gulf against germ warfare agents is a medical roll of the dice, because no single vaccine works against all of the hundreds of microbes that could be used in a biological attack, experts say.

For a military force to be immune to any such assault, the troops would have to be vaccinated weeks or even months in advance and the vaccines used would have to exactly match the microbes used in the attack, the medical authorities say."With most vaccines it takes from two to four weeks to develop adequate immune protection," said Dr. Bradley Sack, a Johns Hopkins University expert on infectious diseases. And often, he said, full protection for some specific diseases could take up to several months.

To develop immunity, the body of each soldier would have to be exposed to a vaccine and then have time to build up the billions of antibodies needed to repel the disease.

If vaccinations of U.S. forces don't start until next week, Sack said, it is unlikely that the troops would all have developed the full immunity needed by Jan. 15, the U.N.-set date for Iraq to pull out of Kuwait or face military action.

U.S. military officials refuse to identify any particular disease for which troops will be vaccinated. But published reports have said that Iraq's biological arsenal may include these diseases: typhoid, cholera, anthrax, rabbit fever and botulism.

This group, says Dr. Sanford Kuvin, vice chairman of the National Foundation for Infectious Diseases, "is only the short list" of possibilities.

"If you restrict the biological warfare possibilities to the short list of common things, it could probably be dealt with in a moderately effective way by the U.S. military," he said.

But the real problem, said Kuvin, is that nature has provided so many microbe warriors that can be marshaled into armies.

"The menu of lethality is very long," said Kuvin. "There are innumerable viral diseases. And I don't know how you could make a cocktail of vaccines that would be effective against them all."

And there are problems even with the "short list."

There is a vaccine for anthrax, the most popular candidate named by experts as a possible biological weapon. But the vaccine is not commercially available and few American doctors, even in the military, have ever dealt with it.

"There's very little of that disease in this country," said Sack.

But another expert, who asked not to be named, said that the military is thought to have developed an anthrax vaccine designed for use in combat situations.

Cholera, another disease candidate on the short list, has virtually no effective vaccine. Sack said there is an experimental oral vaccine that has been tested in Bangladesh, but it has not been tried in this country.

"I've heard that the military may want to use it," he said. "Of the little we have, it seems to be the most effective. It takes at least three doses over a month."

For typhoid, there is an oral vaccine that provides 65 to 70 percent protection with few side effects. It requires four doses over eight days, and immunity is good within 10 days of the last dose, said Sack.

Science has no vaccine for rabbit fever, or tularemia. Neither is there one for botulism, the food-borne bacteria. Both diseases can be fatal, but both will respond to antibiotics.

Kuvin said, however, the biological weapons that are of most concern to him are the hundreds of microbes that are rarely or never seen in the United States, but which could be devastating to American troops.

There are organisms, he said, that spring up from Africa or Asia or South America, wipe out large populations and then burn themselves out, only to pop up again years later somewhere else. They bear names like Blue Nile virus, Rift Valley fever, Marburg virus, dengue fever or lassa fever.

"Some of these are almost universally lethal," said Kuvin. And there is no proven vaccine for many of them.

And even if there were vaccines for them all, which should American troops be protected against?