This year's dominant flu strain has mutated to resist the main weapon against it.
In fact, nearly 100-percent of this year's most common influenza bug is immune to Tamiflu.
That's according to the Centers for Disease Control and Prevention.
Tamiflu's anti-viral drug that stops flu germs from replicating.
The medical community prefers it over the number-two flu treatment, Relenza, because Tamiflu can be taken as a pill or liquid.
Relenza must be inhaled, putting it off-limits to small children and many seniors.
Doctors were warned in December to switch patients to Relenza or combine Tamiflu with an even older antiviral called rimantadine.
Each year, the government tries to identify the influenza variations it thinks will hit hardest, designating two Type A and one Type B strain and cooking up a vaccine to tackle them.
As luck would have it, this year's most common Type A strain is a fairly weak one and there were plenty of vaccines to go around.
Microbiologists say in a worst-case scenario, a long-feared flu pandemic would hit and carry the Tamiflu immunity.
Scientists say the mutation developed randomly and not as a reaction to the drug's overuse.
(Copyright 2009 by Newsroom Solutions)