Autism is a terrible disease with no known cure.
Do you know how many people are affected with it?
Not precisely, but  Autism is caused by both genetic and environmental factors.
how is it caused by enviromental factors?
In some cases, autism is associated with agents that cause birth defects.
I have heard different chemicals such as those in plastic can cause autism.
Perhaps, but Research says genetic factors predominate.
Hopefully science can come up with a way to change this in the future