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Survivors of bird flu, and perhaps other influenza viruses, may not be out of the woods once the fever and cough are gone: Animal studies suggest the virus may damage the brain and cause Alzheimer's and Parkinson's disease.

The tests on mice show that the H5N1 virus can get into the brain, causing damage that resembles Parkinson's and Alzheimer's in humans, the researchers wrote in the Proceedings of the National Academy of Sciences.

"Our results suggest that a pandemic H5N1 pathogen, or other neurotropic influenza virus, could initiate central nervous system disorders of protein aggregation including Parkinson's and Alzheimer's diseases," Richard Smeyne of St. Jude Children's Research Hospital in Memphis, Tenn., and colleagues wrote.

H5N1 avian influenza mostly affects birds, but since 2003 it has infected 436 people and killed 262 of them.

It is an especially destructive virus in people, causing widespread organ damage. Unlike most influenza viruses, it is neurotropic, meaning it can infect nerve cells, including cells in the brain.

There is no evidence the current pandemic strain of H1N1 swine flu affects the brain, although it can cause multiple organ failure.

"Animals infected by H5N1 viruses have demonstrated acute neurological signs ranging from mild encephalitis to motor disturbances to coma. However, no studies have examined the longer-term neurologic consequences of H5N1 infection among surviving hosts," Dr. Smeyne and colleagues wrote.

They infected mice with H5N1, waited until they recovered, and looked at their brains.

"We show that this virus travels from the peripheral nervous system into the central nervous system," they wrote.

The brain was clearly infected, including areas that make the message-carrying chemical dopamine, which are also destroyed in Parkinson's.

Cells died, not enough to cause Parkinson's, but the immune systems of the mice remained activated.

In Parkinson's, the affected brain cells die for unknown reasons but improper immune attack has been one theory.

Clusters of neurological disease sometimes have been reported after influenza pandemics and this study could help explain them, the researchers said.

Perhaps some strains of flu affect the brains of survivors, making them vulnerable to brain-wasting diseases later in life, they said.

"The offending agent may cause a long-lasting immune response in the brain that persists many years after the insult has resolved, leading to a 'hit and run' mechanism where the original insult is no longer present but the secondary sequelae persists," they wrote.