The U.S. government has advised doctors not to prescribe two anti-virus agents to which the influenza virus has shown tolerance.

According to the U.S. Center for Disease Control, a recent test of cold viruses showed that "H3N2," the most common cold virus this year, has tolerance to rimantadine and amantadine.

91 percent of the 120 "A"-type influenza virus samples tested were found to have tolerance to the two anti-virus agents.

The ratio of viruses with tolerance to these two agents was just 2 percent in 2004 and 11 percent last year.