Researchers have tried unsuccessfully for decades to develop drug treatments that would be effective against colds.

While appealing, the idea of preventing the common cold by means of a vaccine is still considered a scientific long-shot. The reason is that there are more than 200 cold viruses, which change rapidly.

In the past year, scientists have identified and cloned some of the cold virus genes -- an accomplishment that has led to the development of two new classes of cold-fighting drugs.

In addition, a team of researchers from the University of Virginia School of Medicine and Johns Hopkins Medical Institutions in Baltimore have discovered that annoying symptoms are not caused by cold viruses, but by the body's reaction to them.

The key ingredients are chemical mediators called kinins, which are naturally produced by the body. When researchers blew kinins into people's noses, they developed cold symptoms: stuffed nasal passages, drippy noses and scratchy throats.

"We may not have to kill the virus to control colds," said Owen Hendley, a University of Virginia researcher. "We may be able to block the symptoms and allow the infection to continue."

That would mean that while sufferers might be infected, they wouldn't feel sick.

Experimental drugs may be able to help researchers accomplish that. One type of drug called ICAM-1 --

intracellular adhesion molecule 1 -- appears to prevent the virus from entering cells in the nose. Win compounds, named after the Sterling Winthrop drug company, which developed them, block the virus from replicating.

In theory, the virus should eventually die out, since it can't reproduce.

Both look promising -- at least in test tubes and experiments with animals. One Win compound is in clinical trials in humans in Rennsalaer, N.Y. But researchers predict it will take years before either compound is on the market.

"We've had a lot of compounds at that stage before," said University of Michigan researcher Arnold Monto. "So I think we have to be very cautious. But certainly the theoretical framework is there."