evidence: The team also measured blood levels of neurofilament light chain (NfL), a biomarker for nerve degeneration. NfL levels increased in people who developed MS compared to those who did not. The increase occurred only after EBV infection and usually before MS diagnosis. This finding shows that the nerve degeneration that accompanied MS did not start before infection with EBV. The researchers say that the association between EBV and MS risk was too strong to be explained by any other known MS risk factors. The findings strongly suggest that EBV is part of the chain of events that leads to most cases of MS. However, EBV in itself is not sufficient to trigger MS. Other unknown factors certainly play a role. "The hypothesis that EBV causes MS has been investigated by our group and others for several years, but this is the first study providing compelling evidence of causality," Ascherio says. "This is a big step because it suggests that most MS cases could be prevented by stopping EBV infection."