Tamiflu, an antiviral used to treat the flu, doesn’t work against most of the virus circulating in the U.S. this season, federal officials say.

It's not a serious problem so far, because fewer than the usual number of people have caught the flu this year, U.S. Centers for Disease Control and Prevention (CDC) authorities say. But 99 percent of cases this year are resistant to the drug, compared to 11 percent last year, The New York Times reports.

The resistance seems to stem from a spontaneous mutation in the virus, not the result of overuse of the drug, according to the Times. They've advised patients to use a Tamiflu rival, Relenza, or an older med called rimantadine, according to Bloomberg News.

For more on why viruses become resistant to medications thanks to evolution—and the prospect of performing DNA analyses of an infection to decide how best to treat it—check out this article in this month's Scientific American.

Image of Tamiflu by Moriori via Wikimedia Commons