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Getting infected with herpes simplex virus one (HSV-1) can increase risk of developing Alzheimer's disease, which is the most common form of dementia, two new studies reported.

HSV-1 is a very common infection that lasts a lifetime. The infection, however, is not always active. When it is reactivated, the infection shows up as cold sores, also known as mouth ulcers. For these studies, the researchers hypothesized that a reactivated HSV-1 increases risk of Alzheimer's because HSV-1 weakens the immune system, which can increase the likelihood that the virus will travel up to the brain where it can contribute to the development of Alzheimer's.

"Our results clearly show that there is a link between infections of herpes simplex virus and the risk of developing Alzheimer's disease. This also means that we have new opportunities to develop treatment forms to stop the disease," said study researcher, Hugo Lövheim, associate professor at the Department of Community Medicine and Rehabilitation, Geriatric Medicine, Umeå University.

In the more recent study, the researchers tracked the mental health of 3,432 participants for an average of 11.3 years. Mental health included memory and dementia. They concluded that people with a reactivated HSV-1 have a two times greater risk of developing Alzheimer's disease.

In the second study, the researchers analyzed samples on 360 patients diagnosed with Alzheimer's disease that were donated to the Medical Biobank at Umeå University. The samples were compared to similar group of people without Alzheimer's. This study also found that people infected with HSV-1 had doubled the risk of developing the neurodegenerative illness.

"Something which makes this hypothesis very interesting is that now herpes infection can in principle be treated with antiviral agents. Therefore within a few years we hope to be able to start studies in which we will also try treating patients to prevent the development of Alzheimer's disease," Lövheim conclude according to the University's news release.

The studies, "Herpes simplex infection and the risk of Alzheimer's disease-A nested case-control study" and "Reactivated herpes simplex infection increases the risk of Alzheimer's disease," were published in the journal, Alzheimer's & Dementia.