The virus, herpes simplex virus type 1 (HSV1), when present in brain, acts together with the type 4 allele of the APOE gene, a known susceptibility factor in Alzheimer disease (AD), to confer a strong risk of AD; in carriers of the other two main alleles of the gene, the virus does not confer a risk. It also has been shown that the outcome of infection in the case of five diseases known to be caused by viruses is determined by APOE. It is hoped that the discovery of the involvement of HSV1 in AD will lead to future antiviral therapy and possibly to immunization against the virus in infancy.