An important antiviral drug no longer works against this season's most prevalent type of flu, which has mutated into a resistant strain, researchers reported today in the Journal of the American Medical Association.

That drug -- sold as Tamiflu in the U.S. -- was one arrow in a very small quiver of antiviral medicines used to battle influenza, an illness that lands 200,000 Americans in the hospital and kills 36,000 every year, according to the Centers for Disease Control.

Public health officials and physicians called the development and spread of Tamiflu-resistant flu disturbing.

"It makes me nervous," said Michael Koller, a doctor of internal medicine at Loyola University Medical Center. "We know that it keeps mutating and that is why it is still around. It manages to figure out ways to outsmart us and our medications."

With Tamiflu no longer effective against this particular flu strain, known as H1N1, physicians are left with one called zanamivir, which has problems of its own. The drug, which is inhaled, is not recommended for some of the very populations that would need it most: Very young children and people with respiratory problems.

For those people, CDC officials recommend a combination of Tamiflu and another antiviral called rimantadine, which was the subject of a flu resistance scare of its own in 2006.

"The problem is that we now have different flu strains that are susceptible to different antiviral agents, and clinicians often do not know what strain is infecting a person. The new recommendations try to account for these different factors," said Alicia Fry, a medical epidemiologist in the influenza division of the CDC and a co-author of the JAMA report.

With so many problems with the antivirals, public health officials are urging people to get a flu shot, which offers good protection this year against the H1N1 strain.

"Flu season is still peaking," said Koller, who noted that he started seeing a rise in flu patients around the first week of February. "There is no sign we are on our way down. It is not too late."

One of the virus' most astounding feats is that it managed to mutate into a resistant form and spread around the globe in a year.

Scientists began picking up hints that was happening during the last flu season, when reports came in from Canada to Russia to Hong Kong to the U.S. of Tamiflu-resistant flu. In the U.S., scientists found that about 11 percent of the H1N1 viruses were resistant.

By summer, South Africa ominously reported all of its H1N1 was resistant. And now that is the case in the U.S. "That is a phenomenal change in one year's time," Koller said. "It is very concerning."