“... their child was completely normal”.
Was it the 18 month MMR? I have read that as well.... I agree that this condition is as elusive as finding a cure for cancer.
Both my friends children were normal until 3. Her son was a happy chatting child until he turned three and slowly lost the vocalization skills he had developed, and then it was all downhill. His sister made it to around two and a half before her Autistic tendencies started to show, but my friend admits they knew what the were seeing with her, so her son may have been showing those signs a little earlier and they didn’t catch them.