I find it easier to think of it like bad eyesight, rather than a disease. We usually think of diseases as things that eventually go away, especially when there are medications for them, and that doesn't happen here.

Some people are born with good eyesight, and they can see everything clearly right from the get-go. Some people can't, and they need the external modifier of corrective lenses or, no matter how hard they try, the world will be blurry. So, they wear those corrective lenses every day just to function in the world.

Taking meds for ADHD is more like that.