It is not yet clear what exactly causes Alzheimer's disease, a progressive disease characterized by memory loss that affects about 5 million Americans. However, findings of two studies suggest that herpes simplex virus, which is responsible for cold sores can elevate a person's risks of developing the condition.

In the first study "Reactivated herpes simplex infection increases the risk of Alzheimer's disease," which was published in the journal Alzheimer's & Dementia on July 17, Hugo Lovheim, from the department of community medicine and rehabilitation at Umea University in Sweden, and colleagues followed over 3,400 individuals who were part of the Betula project, a longitudinal study on aging, memory and dementia, for an average of 11 years, and found that the subjects with certain antibodies to herpes infection were twice likely to develop Alzheimer's disease.

For the second study, the researchers compared the blood samples taken from Alzheimer's patients nearly 10 years on average before they were diagnosed of the disease, with those taken from individuals without the condition. The researchers found that in patients who had their blood taken at 6.6 years before they were diagnosed of Alzheimer's disease, there was an association between infection with herpes and the odds of developing the progressive neurological disorder.

"In the complete sample group, the presence of anti-HSV IgG and IgM antibodies did not increase the risk of AD significantly," the researchers wrote. "In cases with 6.6 years or more between plasma sampling and AD diagnosis, there was a significant association between presence of anti-HSV IgG antibodies and AD."

The second study was published in the journal Alzheimer's & Dementia on Oct. 7.

"Our results clearly show that there is a link between infections of herpes simplex virus and the risk of developing Alzheimer's disease," Lövheim said. "This also means that we have new opportunities to develop treatment forms to stop the disease."

Some experts, however, are skeptical of the researchers' conclusion. Greg Cole from the UCLA Alzheimer Disease Research Center said that a large percentage of the population (90 percent) carries antibodies to herpes and all of them are not likely destined to be struck by Alzheimer's.

Cole, however, noted that an immune response or infection may possibly link herpes and Alzheimer's disease.

"Recent genetic studies have implicated variants of several genes controlling immunity with increased Alzheimer's disease risk. These new results warrant a closer look in larger populations," Cole said.