well, there's actually a debate on that.
they say either a child is born with it, or they get it from a vaccine.
personally, i think vaccine. my 17 year old sister has severe autism. she was diagnosed at age 4. before that, she was very talkative and social. about 4 months before she shut down, she got a series of shots before she went to pre-k. then, the shut down. and now, she's got like an iq or 55. only says 5-10 words, in one-three word sentences. it's sad