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ContextThe “hygiene hypothesis” has implicated sibship as a marker of infection load during early life and suggests that exposure or reexposure to infections can influence the developing immune system. Viral infection has also been implicated in the pathogenesis of multiple sclerosis (MS).ObjectivesTo evaluate whether exposure to infant siblings in early life is associated with the risk of MS, and to explore the possible mechanism for any apparent protective effect, including altered Epstein-Barr virus (EBV) infection patterns.Design, Setting, and PatientsPopulation-based case-control study in Tasmania, Australia, from 1999 to 2001 based on 136 cases of magnetic resonance imaging–confirmed MS and 272 community controls, matched on sex and year of birth.Main Outcome MeasureRisk of MS by duration of contact with younger siblings aged less than 2 years in the first 6 years of life.ResultsIncreasing duration of …