I would like to remind HN that autism being a syndrome, it is likely to be several distinct illnesses with different genetic and environmental factors.
I.e., suppose 50% of people suffer headaches, 50% suffer nausea, and 50% suffer aching back. Further, suppose 12.5% (= 50% x 50% x 50%) of people suffer all three symptoms - in this case, the three symptoms put together are NOT a syndrome.
On the other hand, suppose 40% of people suffered headaches, nausea and aching back together. In that case the three symptoms together are a syndrome.
A syndrome is probabilistic evidence that there is an underlying cause of multiple co-occurring symptoms.
Sorry, I wrote that up fast, if you are interested you can write a reply, I'll try answering tomorrow, it's late for me now.