The oral herpes virus (herpes simplex virus 1 or HSV1) causes cold sores and stays in the body for life. It's very common - most people will eventually get an HSV1 infection. Although it usually infects the body, it also has been found in the brain.
One of the main links between HSV1 and dementia is that HSV1 DNA has been found within the protein clumps that build up in Alzheimer’s disease. Aside from this, some researchers think that herpes could cause excess inflammation in the brain. In lab-grown brain cells, HSV1-induced inflammation changes how well they work. This could then potentially trigger or worsen Alzheimer's disease.
Some population studies suggest that there is a connection between HSV1 and dementia. However, others showed no connection with dementia, only an increased risk of mild memory and thinking problems.
Some people have a version of a gene (ApoE) that increases their risk of Alzheimer's disease (ApoE4). Herpes infection in the brain appears to be more common in people with this version of the gene. One theory is that ApoE4 increases the likelihood of the herpes virus getting into the brain. Some researchers also think that ApoE4 allows more virus particles to attach to brain cells, making it easier to infect them.
Although herpes infections are for life, most of the time the virus is dormant and symptom-free. It is thought that only the re-activation of the virus increases a person's risk of developing Alzheimer's disease. This can be prevented by an anti-viral treatment.
Currently, there is no strong evidence that herpes virus infection causes Alzheimer's disease. The increased presence of the virus in the brain may instead be related to changes to the immune system due to Alzheimer's disease.
