The "hygiene hypothesis" proposes that early life infections may reduce the risk for allergic and autoimmune disorders, and can influence the developing immune system, according to background information in the article. Having siblings may increase the number of early-life infections, and lack of contact with siblings has been associated with several immune disorders. Younger siblings may be important because infants provide a source of common viral infections. Re-exposure to active viral infection is known to cause immune boosting. A protective role for early life infection in the development of MS is consistent with several features of MS, including the apparent recent increase in incidence that has accompanied a decline in childhood infection rates over time.
Anne-Louise Ponsonby, Ph.D., of the Menzies Research Institute, Hobart, Australia, and colleagues conducted a study to determine whether exposure to infant siblings in early life is associated with the risk of MS, and to explore the possible mechanism of any protective effect, such as elevated Epstein-Barr virus (EBV) antibodies. The study, conducted in Tasmania, Australia, from 1999 to 2001, included 136 cases (average age, 43.5 years) of magnetic resonance imagingconfirmed MS and 272 community controls (average age, 43.6 years), matched on sex and year of birth.
The researchers found that increasing duration of contact with a younger sibling aged less than 2 years in the first 6 years of life was associated with reduced MS risk: 1-3 years of infant contact, a 43 percent reduced risk; 3 to 5 years of infant contact, a 60 percent decreased risk; greater than 5 years of infant contact, an 88 percent reduced risk.