By digging deeper, you might find that guy A was stressed out; guy B was suffering from a sinus cold; and guy C had whacked his head against a door. All three shared only the symptom - and not the cause. But all three had something describable, recognizable and treatable as "headache."
Right now, in the world of autism, we are living with a similar situation. People with autism spectrum disorder diagnoses of PDD-NOS, Asperger syndrome or autistic disorder (and even, arguably, non-verbal learning disorder and several other disorders) share symptoms. For all we know, there are a hundred different causes of "autism-like symptoms," just as there are a hundred different causes of "headache-like symptoms." Some may be the result of immune deficiencies. Others may be caused by food intolerances, brain injuries, unique brain structures, genetic anomalies or airborne toxins.
But according to the diagnostic manual, unless the symptoms can be better explained by Retts disorder, Childhood Disintegrative Disorder or Schizophrenia - people with autism-like symptoms have an Autism Spectrum Disorder.
In the past few months, the Vaccine Court has made a number of decisions regarding the claims of parents claiming that vaccines injured their children who now have the symptoms of autism spectrum disorders.
From what I've read so far, it seems to me that the court is actually attempting to distinguish, in its decisions, between "autism spectrum disorders" and "autism-like symptoms." The distinction is based on what the court determines to be the underlying cause of the symptoms. Hannah Poling, for example, had an underlying mitochondrial condition and received too many vaccines in one day. The court acknowledged a causal connection between the vaccines and "autism-like symptoms," but did not call Hannah's condition an "autism spectrum disorder."
But if there really is no distinction in the diagnostic criteria between, say, "autism-like symptoms caused by genetic mutation" and "autism-like symptoms caused by a combination of pre-existing mitochondrial disorder and fever brought about by over-exposure to vaccines," I'm not sure that the court can reasonably say that one is "an autism spectrum disorder" while the other is "a set of autism-like symptoms."
In fact, at least at this point in history (and hopefully all this will change over time!) - if it looks like autism and it acts like autism (and it isn't Rett syndrome, CDD or Schizophrenia) - then by golly, so far as I understand the medical and legal literature, it's autism.