Why are more people coming down with allergies and autoimmune disorders like multiple sclerosis (MS)? One theory is that we live in cleaner environments than previous generations and are exposed as children to fewer microbes. A new study of 400 Australians supports this so-called hygiene hypothesis. Researchers report that test subjects who had the greatest number of younger siblings and who were separated in age from them by less than six years were the least likely to develop MS. Apparently, all the infections they caught from the younger kids helped train their immune system not to attack their own nerves, which is what happens with MS.