Autism is very clearly described in the DSM IV. California and Great Britain have been using this definition since the early 80's, yet the autism rates have gone up at the same rate throughout the US and GB at the same time--from the mid 90's. Therefore, "greater awareness" is not the answer to the explosion. (were they really missing 99.5% of the cases?)
That would depend on whether the diagnosis is regarded as a qualification for services or as a death sentence. When I went to school on this (before the 80s) the textbooks defined only the most severe cases as autism. Our teachers told us that the books were wrong, and that there was a spectrum of symptoms.
Several decades ago I was a family counselor. I had at least one psychiatrist ask me what diagnosis would qualify my client for treatment.