Researchers have found a link between a common virus and Alzheimer’s disease.

Alzheimer’s disease, the most common type of dementia, is a condition that affects the brain, causing changes in it that cause problems with memory, thinking, and behavior.

In a new study published in The Journal of the Alzheimer’s Association, researchers discovered CMV, a type of herpesvirus that often infects individuals during childhood. While the virus usually remains inactive after initial infection, it remains in the body throughout life.

By age 80, about 90% of people have CMV antibodies in their blood, indicating exposure to the virus.

For some individuals, the virus can exploit a pathway called the vagus nerve, which connects the gut and the brain.

If the virus becomes active and travels along this “superhighway,” it can trigger immune system responses in the brain, potentially contributing to Alzheimer’s disease.

How does CMV affect the body and brain

CMV usually remains inactive in most people. It spreads through bodily fluids like saliva, blood and semen but only when active.

In rare cases, the virus remains active long enough to travel from the gut to the brain via the vagus nerve. Once in the brain, the virus can overstimulate microglia, which are immune cells responsible for clearing waste and infection in the central nervous system.

Overactive microglia can cause inflammation and damage neurons, which is linked to Alzheimer’s disease.

A new subtype of Alzheimer’s disease

Lead researcher Dr. Ben Readhead of Arizona State University suggested that the Alzheimer’s subtype related to this virus may be responsible for 25–45% of cases.

This subtype still shows classic symptoms of the disease, such as amyloid plaques and tau tangles, but also has a unique biological profile, including viral presence, immune cell activity, and specific antibodies in the brain.

Evidence from donor studies

Researchers studied tissue from 101 donors, including brain, spinal fluid and vagus nerve. Of these, 66 donors had Alzheimer’s.

In people with the disease, CMV antibodies were found in their intestines, spinal fluid, and brain, and the virus was detected in the vagus nerve.

These findings were confirmed in another independent group and further supported by laboratory tests showing that CMV increases the production of harmful proteins associated with Alzheimer’s.

What does this mean for prevention

It is important to note that only a small subgroup of people with CMV develop Alzheimer’s, despite the majority of individuals exposed to the virus.

Researchers are now working on developing a blood test to detect active CMV infection in the gut. If identified early, these infections can potentially be treated with antiviral drugs, reducing the risk of Alzheimer’s in this group.