The parents in many of these groups, which have ranged from tens to tens of thousands of members, believe that autism is caused by a hodgepodge of phenomena, including viruses, bacteria, fungal infections, parasites, heavy metal poisoning from vaccines, general inflammation, allergies, gluten and even the moon.

The so-called treatments are equally confused. Some parents credit turpentine or their children’s own urine as the secret miracle drug for reversing autism. One of the most sought-after chemicals is chlorine dioxide — a compound that the Food and Drug Administration warns amounts to industrial bleach, and doctors say can cause permanent harm. Parents still give it to their children orally, through enemas, and in baths. Proponents of chlorine dioxide profit off these parents’ fears and hopes by selling books about the supposed “cure,” marketing the chemicals and posting how-to videos.