Viruses that hide in the brain might influence Alzheimer's disease. That is what researchers said in a new study that is expected to restart debate about what causes the brain-wasting disease.

However, the findings do not prove viruses cause Alzheimer's. They also do not suggest that it can be passed from one person to another.

A team led by researchers at New York's Mount Sinai Health System found that some viruses affect genes involved in Alzheimer's. Among those genes are two very common herpes viruses.

The idea that infections earlier in life might somehow begin the process of getting Alzheimer's years later has been discussed for years. But the theory that Alzheimer's starts from sticky plaques that clog the brain has had more support.

The study has some specialists saying it is time to look more closely at viruses. This is because of the failure of attempts to prevent the plaques.

Dr. John Morris directs the Alzheimer's research center at Washington University School of Medicine in St. Louis. He said, "With an illness this terrible, we cannot afford to dismiss all scientific possibilities." Morris was not involved in the new research.

Dr. Rudolph Tanzi is an Alzheimer's specialist at Massachusetts General Hospital. He said the study also agrees with growing evidence that the brain's ability to defend itself against viruses may be riskier than an actual infection.

Tanzi was not involved in the recent study but has performed related experiments with Dr. Robert Moir of Harvard. Their research has shown that sticky beta-amyloid plaque captures germs by completely surrounding them. The researchers say that is why the plaque starts forming in the brain.

"The question remained, OK, in the Alzheimer brain what are the microbes that matter, what are the microbes that start the plaque?" said Tanzi.

The team from Mount Sinai and Arizona State University found some viral suspects — by accident. Their study was not hunting viruses. It was looking for new drugs that would target Alzheimer's.

The researchers were using complex genetic information from hundreds of brains. They wanted to compare differences between people who had died with Alzheimer's and people who had died completely free of Alzheimer's.

The possibility that viruses were around "came screaming out at us," said Mount Sinai genetic expert Joel Dudley. He was a leading author of the research published in the journal Neuron.

The team found viral genetic material at far higher levels in the brains of people with Alzheimer's than in normal brains. Two human herpes viruses, known as HHV6a and HHV7, stood out. They infect most people during childhood, often with no symptoms, and then lie inactive in the body.

Since 1980, researchers have known that some common bacteria and viruses increased the likelihood of getting Alzheimer's. But it was never clear if the germs were bystanders, or actively adding to the development of Alzheimer's.

The new study used computer models to see how the viral genes intervened in human genes, proteins and plaque buildup.

They found a lot of interactions, suggesting that the viruses could even switch Alzheimer's-related genes on and off. To see if those interactions mattered, the researchers created mice that lack one molecule that the herpes virus seemed to destroy. The animals, researchers said, developed more of the amyloid plaques.

Keith Fargo directs scientific programs for the Alzheimer's Association. He said the research makes a viral connection to Alzheimer's much more possible.

But, he noted, the study will not affect how today's Alzheimer's patients are treated.

The National Institutes of Health (NIH) supported the study. The NIH is now giving money for a new study to see if an antiviral drug can help people who have both mild Alzheimer's and different herpes viruses.

Just having a herpes virus "does not mean you're going to get Alzheimer's," Tanzi said. It may not even have gotten into the brain.

But Tanzi added, "The brain was always thought to be a sterile place. That's absolutely not true."

I'm Susan Shand. And I'm Alice Bryant.

The Associated Press reported this story. Susan Shand adapted it for Learning English. Mario Ritter was the editor

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herpes- n. a disease that causes painful spots on the ski

plague – n. a change in brain tissue that occurs in Alzheimer's disease

clog – v. to slowly form a block in (something, such as a pipe or street) so that things cannot move through quickly or easily

impressive - adj. deserving attention, admiration, or respect

scream – v. to shout loudly

symptoms –n. a change that shows that a disease is present

bystander – n. someone who is uninvolved in an incident but comes to watch the police or ambulance

microbe – n. an extremely small living thing that can only be seen with a microscope