The classical description of autism is a normal child who at the age of 8 months to two years develops a high fever. After the fever, he or she deteriorates and develops social problems, ritualistic behavior, speech abnormalities etc.
No one knows why. We do know that rubella can cause it, which is why they are so worried about MMR. Also, measles used to be associated with a lot of encephalitis, that left people brain damaged. That's why a lot of people are worried about MMR vaccine. There is also a question if the problem was the mercury preservative in the vaccine, since heavy metal poisoning like lead poisoning can cause retardation with social abnormal behavior.
Unlike a lot of the paranoia about medicine etc. this one is believed by a lot of doctors. However, most of us feel that the risk/benefit ration is worth it (kids die of measles, and one in ten thousand get encephalitis).
On line book about autism: AUTISM
For parents and relatives of autistic kids: Autism organization