To test the "hygiene hypothesis" -- that infections early in life may reduce the risk of developing allergies and autoimmune diseases in adulthood -- the researchers looked at the life histories of 136 Tasmanian adults with multiple sclerosis and compared them with 272 without the disease.

Those who as children had up to five years of contact with a younger sibling had an 88 percent reduced risk of developing multiple sclerosis. Those with one to three years of exposure had a 43 percent reduced risk.

Longer exposure to a younger brother or sister also translated into a lower risk of developing mononucleosis and lessened the chances of an exaggerated immune response to the Epstein-Barr virus. If infection with the virus does not occur until adolescence or early adulthood, it causes mononucleosis up to half the time.

Children up to age 2 are magnets for common viral infections, including Epstein-Barr, so infections passed to their siblings help the older children's immune systems develop, the researchers said.

Autoimmune diseases are caused by an overactive immune system attacking the body's own tissues -- in the case of multiple sclerosis, the sheathes that protect nerves.

Further research was needed to confirm the findings published in this week's issue of the Journal of the American Medical Association (news - web sites), wrote study author Anne-Louise Ponsonby of the Menzies Research Institute in Hobart, Tasmania.

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