ROBYN O’BRIEN likes to joke that at least she hasn’t started checking the rearview mirror to see if she’s being followed. But some days, her imagination gets away from her and she wonders if it’s only a matter of time before Big Food tries to stop her from exposing what she sees as a profit-driven global conspiracy whose collateral damage is an alarming increase in childhood food allergies. Ms. O’Brien has presented her views, albeit in a less radical wrapper, on CNN, CBS and in frequent print interviews. Frontier Airlines and Wild Oats stores distribute the allergy-awareness gear she designed....