Common viruses associated with lower pediatric multiple sclerosis risk.

In this study of paediatric multiple sclerosis. the Epstein Barr virus was associated with risk:The effects of herpes simplex depended upon certain genes : HSV-1 was associated with an increased risk of MS in those without a DRB1*15 allele , but with a decreased risk  in those who were DRB1*15-positive.
Cytomegalovirus infection was asociated with decreased risk. This has also been reported for chicken pox in paediatric studies while hepatitis B infection has also been reported to be protective in adults.
See MS risk factors
Clearly, the effects of viruses are complex and gene dependent (and vice versa) and whether or nor a risk factor is a risk factor depends on which genes we have.