At this point in time we do not know what causes autism.
However, current research indicates that anything that
can produce structural or functional damage in the
central nervous system can also produce the syndrome of
autism. We know that certain viruses and known genetic
conditions have been associated with autism. In addition,
in rare cases there is more than one autistic child in a
family. At this point, it is believed that about ten
percent cases of autism can be accounted for genetically.
Generally no one can tell you specifically why your child
is autistic or why you have an autistic child and your
neighbor does not.