I did prefer to avoid mercury in vaccines for my first child ... I have got less fussed since then, mind you! I think it will turn out to be a complex interplay of genetics, pollutants of various types, issues affecting gut flora (birth mode, feeding method, age of weaning, not too early, introducing foreign proteins while still being breastfed) I suspect the link to antibiotics and paracetemol is more link than correlation - a child destined to have asthma may have courses of both when young due to being prone to infections