Autism is a neurological difference.
This difference can cause disability. For example, sensory processing delays can impair communication and perception, or awareness and movement of the body in physical space. Or, an autistic person may not be able to speak because of their neurological make-up. There are many more, sometimes compounded, disabilities that are often present in autistic persons.
This neurological difference can also have debilitating effects, like anxiety, which when chronic, can be considered both as an illness and a disability.
The consequences of this neurological difference can cause a very variable set of disabilities and illnesses.
But autism, in itself, is neither an illness or a disability.
It is not a physical illness, although many autistics suffer from various illnesses that do have a higher chance of occurring in an autistic person.
It it not a mental illness, although many autistics suffer from anxiety and depression.
It is not a disability in itself, although many autistics have major physical and sensory challenges, and are disabled as a result.
Autistics are just regular people trying to live their lives as well and as happily as they can, each with their own challenges and strengths. Some have disabilities, some have illnesses. Some have neither. Some live with both.