The hygiene hypothesis says, basically, that people get immune disorders because their immune systems develop in overly clean environments. It's an attractive hypothesis, harking back to the days when kids rolled in the mud, white-washed fences barefoot, got lots of intestinal parasites, and generally did something besides sitting in germ-free rooms playing video games all day. And there's some evidence to support the hypothesis; several studies have suggested that children who grow up on farms are less likely to have allergies, for example. Although it's related to different immune cells than those involved in allergies, multiple sclerosis is also a disease of the immune system. For this study, researchers looked at whether being exposed to infant siblings early in life was related to the risk of multiple sclerosis. Having younger siblings is often used to estimate whether people were exposed to many childhood infections.

What the researchers wanted to know: Is having contact with babies when you're young related to risk for multiple sclerosis?

What they did: The researchers recruited people with multiple sclerosis (MS) in Tasmania, Australia. They got 136 to join, which they think is three quarters or more of the MS patients who were eligible for the study. Then, for each patient, they randomly chose two controls of the same age and sex as the patient from the voter lists (in Australia, voting is compulsory). The researchers compared the patients and the controls to see if MS patients were less likely to have had younger siblings, as they predicted. For this study, infants were up to age 2.

What they found: The more younger siblings a person had, the lower the risk of multiple sclerosis. Someone who'd had one or more "infant-years" of exposure (which could mean one year in the presence of one infant sibling, or several years of different younger siblings added up) by age 6 was only about half as likely to have MS as a person who had less than one infant-year of exposure by age 6.

What the study means to you: These findings lend support to the idea that there are benefits to giving an immune system lots of interesting experiences early in life.

Caveats: In case you're wondering—no, having more children probably isn't a good way to protect your kids from MS. Eventually, one of them has to be the youngest kid. And this is probably just one of many genetic and environmental factors that contribute to multiple sclerosis.

Find out more: The National Multiple Sclerosis Society has a lot of information on multiple sclerosis, including this sourcebook about the epidemiology of MS.

Read the article: Ponsonby, A.-L. et al. "Exposure to Infant Siblings During Early Life and Risk of Multiple Sclerosis." Journal of the American Medical Association. Jan. 26, 2005, Vol. 293, No. 4, pp. 463-469.

Abstract online: http://jama.ama-assn.org