No-one really knows. I’m sure there’s a Nobel Prize in it for anyone who can identify a single cause of autism! It is clear that autism is a very complex condition which most likely has a variety of causes rather than a single one. You can find more detail about the current thinking on causes and genetics of autism on the National Autistic Society’s website.
The most important thing is to accept people with autism for who they are and to recognise that they have strengths as well as weaknesses. They will often need help to cope in a world that is mostly populated by ‘neurotypical’ people but that doesn’t mean we should try to stop them from being autistic. Autism is as much part of a person as their personality. Speech and language therapists (and other professionals) work with people who have autism to help them develop strategies that allow them to function in the world around them. (See FAQ ‘How do speech & language therapists help people with autism?‘)
It is important to understand that there is no scientific evidence that the MMR vaccination causes autism. Dr Ben Goldacre, who is a medical doctor specialising in research into epidemiology, talks about how the media fuelled the MMR scare in this extract from his book, Bad Science.