I think they overlap? Illness, developmental differences, chronic conditions, disability. And often people have more than one of whatever we call it.

I'm allistic; most of my life I've had good mental health but I had postnatal depression after my last pregnancy and my mental health is still rocky. I also have chronic physical and sensory processing conditions which I need to manage, but don't (yet) greatly limit my daily activities.

My older child is autistic and seems to be in good physical and mental health generally. (As best as I can judge, and I might be wrong.) I think I'm reacting badly to regarding my child's autism as an illness rather than a difference?