As children or young adults, most people come into contact with the virus that causes cold sores, herpes simplex virus type 1 (HSV-1). After the first infection, the virus stays in the body for life, hiding quietly in the nerves—but scientists have found that this same virus may also be linked to something much more serious: Alzheimer’s disease.

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Research suggests that the virus that causes cold sores could increase the risk of Alzheimer’s by 80 percent in some people.

Over 30 years ago, a team of scientists discovered that HSV-1 could be found in the brains of older people, a surprise since many believed the brain was protected from germs thanks to the blood-brain barrier. Now, a researcher on that same team announced this summer that people with one particular Alzheimer’s risk factor, a gene called APOE-e4, have a risk that is many times greater if they have also been infected with HSV-1.

To test the theory, the team studied brain cells that they had infected with the virus, and determined that those cells developed amyloid and tau, abnormal proteins found in the brains of people with Alzheimer’s. They theorized that the virus stays asleep in the body for many years, but as people get older and their immune systems weaken, the virus may wake up in the brain. When it does, it can damage brain cells and cause swelling, which could lead to Alzheimer’s in some people.

The researchers also found the virus’s DNA inside the sticky protein clumps in Alzheimer’s brains. When scientists used antiviral drugs in the lab, the damage was reduced, suggesting that medicine might one day help slow or even prevent the disease.

A 2025 BMJ study came to similar conclusions. It suggests that people who have had a previous case of HSV-1 are 80 percent more likely to develop Alzheimer’s later in life, even after controlling for risk factors like carrying two copies of the APOE4 gene.

To assess the role of HSV-1, the researchers grouped the medical records of over 300,000 people aged 50+ into two evenly split cohorts: those who had been diagnosed with  Alzheimer’s disease, and those who had not. They found that 1,507 (0.44 percent) people diagnosed with Alzheimer’s disease had previously experienced an HSV-1 infection, compared with 823 (0.22 percent) people who were not diagnosed with Alzheimer’s.

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Medications and vaccines could curb the risk.

Bolstering the BMJ researchers’ insight, they also determined that people who had taken medication to treat their HSV-1 infection were 17 percent less likely to develop Alzheimer’s than those who did not.

Researchers have also looked at the shingles virus, which is used to treat another herpesvirus: varicella-zoster virus (VZV). Though the vaccine is not used to prevent HSV-1, they saw relevance in the fact that people who got the shingles vaccine were less likely to get dementia later on.

As Best Life previously reported, a 2025 study published in the journal Nature found that people who received the shingles vaccine Zostavax (an older version than what’s currently offered today) were 20 percent less likely to develop dementia within the next seven years than those who didn’t get the shot.

Preventing the spread of HSV-1 is a challenge since it is very common and often spread through saliva, but you can take steps to reduce your risk. To avoid infection, avoid close contact, particularly oral or sexual contact, with someone who has an active outbreak of oral or genital sores.

And, if you do contract HSV-1, talk to your doctor about whether treatment might be right for you. Not only will this hasten the healing of your cold sore, but it could also protect your brain for years to come.

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