A few years ago scientists observed something interesting: people who grew up on a farm had a much lower incidence of allergies and asthma. They speculated that rolling around in the dirt, exposure to animals, etc. exercised the farm kid's immune systems from an early age, and that delivered a lifelong benefit. Back in the '80s, scientists discovered (to their vast surprise!) that peptic ulcers weren't primarily caused by stress, but rather by a bacterial infection of the stomach. However, most people carrying this bacteria don't develop ulcers; their immune systems fight it off.
Now scientists have discovered a link (not proved causal yet) between the lack of certain “good” intestinal bacteria and autism. We get “infected” with these intestinal bacteria through things we ingest (eat). We like to think that we ingest only our food, but any parent can tell you that kids will ingest all sorts of non-food items. For farm kids, that includes things of a much wider variety than the typical city kid. Scientists studying autism have noted that the incidence amongst farm kids is significant lower. Which leads me, naturally, to speculate that the farm kids' wider, er, ingestive range increases the chances that they'll be infected with the good intestinal bacteria – and therefore have a lower propensity to autism.
Wouldn't that be interesting if autism turned out to be as easy to deal with as peptic ulcers turned out to be?