Comment by alterom
Pretty big. The key word is "comorbidity"; having one of them means you are more likely to have the other than a random person.
There's also an overlap in traits.
I'm AuDHD (autism + ADHD). You can read about my ADHD side if the experience (with memes!) here:
What a weird word for things that are non-lethal. Why don't they just call it correlation?