FRIDAY, Oct. 24, 2014 (HealthDay News) -- The virus that causes common cold sores -- herpes simplex -- might increase the risk of Alzheimer's disease, two studies by Swedish researchers suggest.

In fact, being a carrier of certain antibodies to the virus can double the risk of Alzheimer's disease, the researchers found.

"The identification of a treatable cause [herpes simplex] of the most common dementia disorder is a breakthrough," said lead researcher Dr. Hugo Lovheim, an associate professor in the department of community medicine and rehabilitation at Umea University in Sweden.

"Whether treatment of herpes infection with antiviral drugs may slow the Alzheimer's progression is not known, but is certainly worth investigating in clinical studies," he said.

But others aren't so sure that there's a clear cause and effect relationship between herpes simplex and Alzheimer's disease.

Dr. Sam Gandy, director of the Center for Cognitive Health at Mount Sinai Hospital in New York City, doubts that herpes and Alzheimer's disease are connected.

"From time to time data such as these appear in the literature, but they do not address causality or mechanism. The new data are likewise not definitive, and they do not say anything new about the association," he said.

"I do not disbelieve the data. I simply do not know whether the association has anything to do with the cause of Alzheimer's disease," Gandy added.

Herpes simplex is a common infection, affecting up to 90 percent of the population, according to background information in the article. It's the virus that causes cold sores, according to the American Academy of Dermatology (AAD). Once you have herpes simplex, you have it for life, though the virus isn't always active, according to AAD.

The theory linking the herpes virus and Alzheimer's disease is that the virus weakens the immune system, allowing the virus to spread to the brain, which may start the process toward dementia, the researchers said.

In one study of nearly 3,500 people followed for an average of 11 years, Lovheim's team found that having certain antibodies to a herpes infection doubled the risk of developing Alzheimer's disease.