ASD is a set of symptoms. There’s likely many causes. If it were commonly caused by infections of the gut people would have noticed more prevalently as anti fungal and antibiotic treatments aren’t uncommon.

Yeah, probably related to gut microbiota.

Yes, stool transplants attempt to reset the gut to a normal set of flora. The hypothesis is that something upset the flora in the gut of kids with ASD and resetting it resolves the symptoms. When I did research in the field, the thought at the time was that various Clostridia species may be the cause. They are known to produce toxins that can act on neurological cells (think Clostridum botulinum). The thought was these bacteria were more prevalent in kids, and their presence released toxins causing the symptoms. By resetting the gut and getting rid of these bacteria, the toxins disappeared and so did the symptoms. I haven't look at the literature in quite a while so things could have changed.