Attention is something that can be measured with tests. People who score significantly below average have some disability causing them to score below average. It might not be a disease per-se, but it is a dysfunctional behavioral characteristic. Stimulants measurably increase a persons ability to maintain attention on such tests, leading to higher scores. If the score is heightened such that it falls within or above the average range, then the dysfunctional behavioral characteristic is essentially treated.