Good exposure: contact with babies might lessen MS risk.

People who grow up with younger siblings close to them in age are less likely to develop (MS) than are people without such siblings, a new study finds.

The finding supports the idea that sloppy kisses from baby brothers and sisters might fend off disease in later years. As such, it adds to evidence in support of the hygiene hypothesis, which holds that early, frequent exposure to infectious agents prepares the immune system to fight off diseases rather than to turn against a person's own tissues, as occurs in autoimmune diseases, say Anne-Louise Ponsonby of Australian National University in Canberra and her colleagues.

Most researchers consider MS--which is marked by tremors, pain, loss of muscle coordination, and slurred speech--to be an autoimmune disease. It appears when fatty sheaths that insulate nerve fibers are damaged, as if by an errant immune attack. What would trigger such an immune misfire is unclear, but scientists suspect some combination of a person's genetics and exposure to pathogens.

The new study suggests that childhood exposure to infants and the infections they spread could reduce the risk of a person generating a self-destructive response when reexposed to these infections later, Ponsonby says.

In the Australian state of Tasmania, Ponsonby and her colleagues identified 136 people with MS and 272 others of matching age and gender who didn't have the disease. All participants gave blood samples, and each one or a close relative provided details of the participants' childhoods.

For each study volunteer, the researchers calculated the number of preschool years--up to age 6--accumulated with a sibling age 2 or younger. Years with each baby brother or sister were counted separately and then added together. Those participants who had accumulated more than 5 such years had a risk of MS only one-eighth as great as that faced by a person with less than a year of such contact.

People who had had 1 to 5 years of preschool contact with baby siblings had roughly half the chance of later developing MS that people with less exposure did, the researchers report in the Jan. 26 Journal of the American Medical Association.

The participants averaged age 44 at the time of the study. When they were children, there was little day care in Tasmania, suggesting that most preschool contact was with family members, the authors say.

The infectious agent most often linked to MS is Epstein-Barr virus. When contracted in early childhood, the virus causes only mild, flulike symptoms. But if a person gets an Epstein-Barr infection after the onset of adolescence, the virus can cause infectious mononucleosis. Nearly all the study participants had been exposed to the virus at some point, blood tests showed. But the MS patients were more than twice as likely as the others to have had mononucleosis.

The study provides "further confirmation of an important role for [Epstein-Barr virus] in MS," says Alberto Ascherio of the Harvard School of Public Health in Boston. "It seems that it's better to be infected ... early in life than during adolescence."