The virus implicated in Alzheimer's disease, herpes simplex virus type 1 (HSV1), is better known for causing cold sores. It infects most people in infancy and then remains dormant in the peripheral nervous system (the part of the nervous system that isn't the brain and the spinal cord). Occasionally, if a person is stressed, the virus becomes activated and, in some people, it causes cold sores.
We discovered in 1991 that in
many elderly people HSV1 is also present in the brain. And in 1997 we showed that it
confers a strong risk of Alzheimer's disease when present in the brain of people who have a specific gene known as APOE4.
The virus can become active in the brain, perhaps repeatedly, and this probably causes cumulative damage. The likelihood of developing Alzheimer's disease is 12 times greater for APOE4 carriers who have HSV1 in the brain than for those with neither factor.