Unfortunately we don’t have all the answers to autism yet.
We do know that autism has a strong genetic basis. So it does run in families to some extent, but its not a simple pattern. It is not yet known whether it is cause by lots of different rare genetic changes different in each person, or by complex combinations of common genetic changes that are shared between sufferers. There are scientists at my institute who are studying thousands of people with autism to try and get to this answer.
There may also be “environmental triggers” that can increase your chances of developing autism if you have some of the genes. We don’t know what they are either. Some people have claimed pesticides or childhood vaccines are triggers, but the vast majority of scientists do not accept the evidence for that. There was a big scandal a few years back when a doctor got in a lot of trouble because his studies into the dangers of vaccines proved to be false.
So we have lots to learn, but I think our recent advances in genetics will eventually point us to the genes involved. Maybe in the next 5 years or so.
Autism is a really interesting condition – no two people with it have the same set of symptoms or behaviours. As Darren says, one of those things where we’ve got a lot to learn both in figuring out how it occurs and how to work with people who have it. The last one’s a big challenge for teachers.