Nature news reports an unpublished meta analysis of Cesarean section and asthma risk. The authors interpret the outcome in the light of the hygiene hypothesis: unhygienic siblings, risky! Normal delivery, risky!! No early bullshit, risky!!!
Again, delivery mode may be a proxy for physician contact and iatrogenic causes. Having heard today also lectures that take the hygiene hypothesis for granted, the old adage comes to my mind:
You may fool all the people some of the time. You can even fool some of the people all of the time. But you cannot fool all of the people all the time.