Being like

Autism is not in any meaningful sense a spectrum: it's a cloud, and one in greater than four dimensions, overlapping a large number of other conditions and involving handwaving both in the definitions and the symptoms.
Mathematically, if you analyse the DSM-IV definition (two from column one, one from column two, etc) and do the perms and combs, there are about sixty-five thousand different ways to be officially autistic before you begin to address spectrum-related measurements such as severity.
Autism is a lot less like "having tuberculosis" and a lot more like "being like Hamlet" (or Heathcliff, or Mr. Pickwick, or Rasolnikov, or any literary figure) than the general public believes. Which makes clocking its incidence problematic, and deciding whether ABA is effective particularly problematic.

Commentary on discussion of Autism at Crooked Timber.