Child abuse of the intellectually disabled is much easier because of their lack of understanding. We don't always see the abuse, nor hear about the abuse from the child or adult with Down Syndrome, or other intellectual disability. So there are two things we need to look at to try and prevent or stop the abuse of those with Down Syndrome:
One of the first signs of abuse is a change in behavior. Often the adult or child with Down Syndrome will act out the abuse they recieve. By noting changes in behavior, areas of abuse may come to be noticed that would have remained hidden.
The next line of abuse protection rests with the Down Syndrome or other intellectually impaired person, in that they need to be educated / trained about what is right or wrong beghavior and what to do should the situation arise.
Down syndrome is one of the leading clinical causes of cognitive delay in the world - it is not related to race, nationality, religion or socio-economic status. Why does it happen? Currently scientifically unexplained.