Autism is a term that refers to a variety of neurological developmental issues, and that affects certain areas of the brain, the ones that are responsible for the language skills, the interaction with others, and many more, depending on each individual. People that suffer from autism are acting differently from each other, and because we know so little about the brain and the way it works it's hard to understand what causes this affection or what is the best method of treating it.
Most of the doctors are convinced that the causes of autism are genetic and are located in the brain only, because many tests were performed on autistic persons, and their organisms are identical to the organisms of normal persons, so everything that has to do with this condition is in the brain.
In the beginning, when autism was first discovered in 1943 and there were very few cases that could be studied, people thought that autism was caused by parents who raised their children wrong, did not show them love and treated them bad, but this conception was proved to be wrong soon. However, nobody discovered a certain cause of autism until this day, so the reason why autistic persons are different is unknown. There are some theories about what could be causing the disorder but nothing is certain.
Recent research has linked chromosomes with autism, and they may be guilty of something because in many other affections of the brain, like the down syndrome, chromosome anomalies are the cause. But if you compare the genes of an autistic with the genes of a normal person you aren't going to see any difference, so it can't be said that a certain gene or chromosome is guilty.
A large number of scientists also believe that some children are born with some anomalies and predispositions in their genes, and these combined with several environmental factors cause autism to appear. Probably not a single gene is not normal, but more of them are slightly different and when the environmental factors appear too, a 'button' is pushed and the disability appears.
Other theories suggest that a severe emotional trauma may affect the brain so much that the victim withdraws from society and other people.
Until the future comes and medical science evolves to the point where the causes of autism are certain, we can only suppose why it appears. Unfortunately, because we don't know the causes we also don't know how it can be cured. For now, we can only hope, and help the autisics as much as we can.