I've had this question posed to me a thousand times. I know I've blogged on it before, but it seems to bear a rewrite as information evolves.
Autism actually comes from several sources, enough that it's very difficult to trace all forms. Autism is most largely blamed on mercury in vaccines. That only covers about two-fifths of the autism community however. Speciality groups will act as if their approach will solve all issues for all forms of autism, but they can't. That's scientifically impossible. Other groups only cater to one form or branch of autism and leave the rest out.
Now, in talking about where autism comes from, I'm going to mention the cause as forms of damage to our genetic structure. That is in no way meant to imply that people with autism are "damaged" or "useless" as a whole. That is not to imply that all people with autism need to be "cured". We do live with a scale of challenges from having autism and it would be folly to ignore them or consider them for anything less than what they are. It's too easy for us to forget that autism is different for everyone. That the effects are a spectrum of light to heavy effects. People of the high functioning areas may go through life with little difficulty while others can barely function at all. So if you are reading this and are fortunate enough to be high functioning and don't consider your conditions damaging at all, this article is not accusing you or anyone like you of being "damaged". You don't have to be "cured". No one can force you to be.
My definition of damage: How about impaired social function and erratic sensory issues? Those aren't fun and to me they can be very damaging. They aren't the way our bodies are supposed to work. When something doesn't work like it's supposed to, it's "damaged". I am not damaged as a whole person, but parts of me don't work right causing significant life challenges. I would love to have my sensory integration problems "cured" so that simple sounds don't bring me to my knees around "normal" people barely affected.
Now that I've finished the great big disclaimer, what are the sources of autism?
Lead, mercury, and a score of long time pollutants are big time suspects. Even aerosols, after all they could make a hole in the ozone right? How much of that have we breathed in over generations? In what they called "disintegrative child disorder" a child would be functioning normally and then suddenly lose significant function. Function that includes talking, feeding themselves, and loss of potty training. This has been attributed to vaccine "poisoning" or damage. There are long term affects that I believe should be viewed as suspect for longer term damage over generations of exposure or use. Work with dangerous chemicals, drug and alcohol use, heavy pollutant exposure, lead exposure, and even smoking tobacco over generations has slowly damaged our genetic structure. In short, we've created "flaws" that have become hereditary. These aren't just suspect for autism, but a wide range of disabilities that can be hereditary.
Some forms of autism, such as Asperger's syndrome, have been found to be hereditary in nature. That means it has gone so far as to become a genetic trait that can be copied in our children. I believe this is a large part of why numbers are increasing. The human race can't just stop having kids you know. I am going to go so far as to predict that numbers will rise by 50% in the next generation. That's because the average family has two children. Even if one child doesn't have autism, they can still carry the trait from their parents and pass it on to their children.
Sometimes, other injuries can cause damage that mimics autism or exacerbates existing conditions. I'm talking mostly about head injuries. Severe psychological trauma can also worsen some conditions. Sometimes permanently.
I also believe that the nearly limitless combinations of altering conditions over generations, that are different for each person or family tree are why there are so many different aspects on the spectrum of autism. I think it's reasonable to consider that autism through a family caused by drug use combined with lead exposure (over simplifying here) can have a few different effects than a family with causes from totally different chemical environments and exposure. Imagine hundreds of thousands of possible combinations of environmental exposures, each with their own hand in autism. No wonder it's so different for everyone. No wonder it's a "spectrum". No wonder it can't be effectively traced.