Depending on the severity of the autism, some children may be mentally handicapped, although many people with autism are intelligent and just cannot communicate and express themselves.
They often have other illnesses that co-exist with autism such as a sensory processing disorder like hyperacusis (hearing everything too loudly), immune system dysfunction and gastrointestinal disease.
Some children have a very mild type of autism in which they are intellectually normal but they feel awkward in social situations, don’t know how to have appropriate conversations and don’t enjoy playing with other children. They can struggle with school and with relationships because of these issues but otherwise lead normal lives.
Autism used to be a rare disorder, first identified in 1943 by child psychologist Leo Kanner and named Kanner Syndrome. He also referred to it as infantile autism as it occurred in infancy and often at birth.
https://www.empowher.com/autism/content/introduction-autism-and-its-prevalence