I've read most of this thread (17 of the pages anyway) and feel fairly compelled to reply.
I know 'credentials' shouldn't be necessary but just so you know where I'm coming from, I'll throw them out here.
I've worked with children with disabilities for nigh on 25 years, 60-70% of whom have had ASD of some type or presentation. I have 3 children of my own, none with ASD.
I play more cards on the table before starting; I'm basically in agreement with MSO, while I think the way he/she has argued has been unhelpful, the basic points about evidence/research/Wakefield I would assert as correct.
My post is focused on 2 things however, firstly history and prevalence.
There has been mention, in this thread, sorry not time to go back and link though I think in connection with the Triad of Impairments, that there were no such conditions before vaccinations. That thinking is inherently flawed, sorry. Although Autism was first defined as an isolated condition in the 1940's that in no way means it didn't exist until then, or just before then. There's no way of measuring prevalence of a condition before its been defined, in addition Autistic as an adjective was used (erroneously we now know) in Schizophrenia in the late 1800's / early 1900. Indeed, adults with learning disabilities who I worked with in the early 1990's still had diagnosis as children of "childhood Schizophrenia". They are clearly now considered, and indeed diagnosed, as being on the Autistic Spectrum. In fact, and I'll now descend into the trap of using anecdote, my first placement when training in the late 1980's, was in a 'unit' where 14 adults lived. Out of those 14, 1 was diagnosed as having ASD. Now (as I still have contact with them or people who support them,) 12 have a diagnosis of Autism. They haven't developed Autism in their 30's, 40's and 50's, they always had it, it was just sooo poorly diagnosed even as, comparatively, recently as the 80's and 90's. In addition, however poor that lack of diagnosis was, learning disability services were actually better, if that can be believed, at recognising Autism than mainstream services.
Autism has always been around, but like people have already said, it manifests in a myriad of different ways "Autistics; they don't give eye contact!" which many of us will know is true of some, but a ridiculous generalisation.
Many 'lay' people and that's mostly everyone prior to having a child diagnosed with Autism or supporting people professionally, would be hard pushed to recognise Autism unless it was in Rainman. Parents I work with will tell me that many of the difficulties they face are because 'the public' doesn't recognise their child as having a difficulty/disability, they just see a child behaving 'weirdly' or being 'naughty' Even with this so called epidemic, lay people still have no awareness.
I am in just no way convinced that the prevalence is increasing, diagnosis is, criteria is, the spectrum is. As I've said, people who just 10-15 years ago wouldn't have been diagnosed as having ASD, now are. Adults with disabilities are now being re-diagnosed after years of inappropriate/poor support services (and not to prove against the childhood increase agrument, but to ensure those people are given the support/care they need.)
I'll finish with one word re: MMR/Vaccines and regression. I have close experience of a child with Autism. His mum was convinced that the MMR was responsible. I can, hand on heart, say I had spotted signs of ASD 12 months before the MMR. I don't say that to criticise his mum, or any parent convinced of the cause of their child's ASD, and, again its anecdote, not proof.
Regression is complicated. My view is that ppl with ASD regress post ASD diagnosis as well as before, meaning that, it is part of the condition as well as the start of the condition. Many of the features of someone's ASD can go in cycles of severity, sometimes triggered by external factors, sometimes not. My view, for what its worth and with no data at hand to back it up, is that for those with an initial drastic regression, is that they were born with ASD. That initial regression being part of a shutdown in relation to a number of factors; ill-health (before a vaccine shout, it could as well be measles as a cold, flu, a virus - I've no many children with ASD (post-diag) regress when unwell, sometimes recovering sometimes not) or as a response to the developmental stage they're at; the move from being a recipient of care and love and support to being encouraged but he world around them to be more empathic,participant in the world. That pressure causes a crash. (I've not explained that well, I could prattle on for pages about it, and more empirically too)
As for bowel problems and Autism... no time; work!