As if cold sores weren’t bad enough, herpes simplex virus type 1 may now be linked to Alzheimer’s disease. The connection involves apolipoprotein E (apoE), a protein that helps to transport cholesterol through the body. There are several types of apoE genes; one, APOE-e4,is the leading risk factor for developing Alzheimer’s. In previous studies, researchers also found that people who have the APOE-e4 gene and have herpes simplex DNA in their brains are even more likely to be afflicted with Alzheimer’s.

To probe the link between gene and virus, neuroscientist HowardFederoff, of the University of Rochester School of Medicine, and his colleagues created mouse-cell cultures that expressed either theAPOE-e4 gene or another variant (APOE-e2 or APOE-e3), then infected the cells with herpes. While all the cell types could be infected, the virus was much more likely to be active in the cells expressingAPOE-e4. “There is something about APOE-e4 that may control the way the virus decides if it is going to be more or less active,” Federoff says. An active virus means cell injury or death; a dormant virus poses no threat.

How that might eventually predispose a person to Alzheimer’s is still unclear, but it may involve a receptor molecule for the herpes virus in the connections between nerve cells. “The failure of the electrical connections, which would cause cognitive impairment, might be linked in some way with the herpes simplex virus,” Federoff says.But he warns, “The last thing I would want people to believe is that if they have the herpes virus or frequent cold sores, they are definitely going to get Alzheimer’s. The virus represents just one possible factor.” Medications are available that can prevent reactivation of the virus—and cold sores. But if it’s necessary to prevent herpes simplex infection in the first place to ward off Alzheimer’s, “it could spawn an effort to develop prophylactic vaccines,” Federoff says.