In 1987, I started doing music therapy with children who had High-Functioning Autism (HFA). Although we didn't call it HFA at that time, we knew that some autistic children were higher functioning than others. I have always said - and continue to believe - that if you have an undiagnosed HFA child WITHOUT any comordid conditions, you have a child who may go his or her entire life without ever being diagnosed with HFA. The child might be viewed as a little weird by peers, but without any comorbid conditions, few - if any - adults (e.g., parents, teachers, etc.) would ever suspect that the child had HFA. This is because HFA has few problematic symptoms in-and-of itself. Most often, it is the conditions associated with HFA that indicate something is not quite right. For example, an alarming number of children who were eventually diagnosed with Asperger's were first diagnosed with ADHD years earlier.
Unfortunately, in my 25+ years of experience, I have never met a child with HFA or Asperger's that did NOT have at least one comorbid condition. But, the good news is that most comorbid conditions are very treatable.