It's rather similar to the incidence of dyslexia. When I began teaching, the word was unknown, but I did come across pupils with typical dyslexic-type reading problems. Much later in my career, we heard about research and findings, whereupon many, many pupils were found to be dyslexic. I don't think the causes of dyslexia have yet been ascertained, and I doubt whether the incidence of it has increased over the years, merely more accurate diagnosis. I did know two children in my local area (when I was a child myself) who in hindsight appeared to be severely autistic. They both went to a special 'unit' each day, but lived at home and we often saw them outside with their parents, (two different families.) Of course, the triple vaccine was unknown then, so it could not have caused these two to be affected.