Epstein–Barr virus (EBV) infection in adulthood is a well-established risk factor for multiple sclerosis (MS), and new research indicates that human herpesvirus 6A (HHV-6A) further increases the risk of developing the disease. Grut et al. tested serum samples from 670 presymptomatic individuals who subsequently developed MS and 670 matched control individuals. In participants over 24.9 years of age, coexistence of antibodies against the EBV protein EBNA1 and the HHV-6A protein IE1A was associated with a higher risk of MS than seropositivity for either antibody alone. The findings indicate an additive effect of the two viruses on MS risk, and the researchers postulate that HHV-6A infection could induce reactivation of EBV.