A disabled person (child or adult) is someone who has a physical or mental impairment, which has a substantial and long-term adverse effect on his or her ability to carry out normal-day-to-day activites.
Someone with impairment may be receiving medical or other treatment which alleviates or removes the effects of that impairment (but not that impairment itself). In such cases the treatment should be diagnosed and the impairment is taken to have the effect it would have had without the treatment.
Some people are automatically deemed to have a disability covered by the Act - those with HIV, cancer, MS and severe disfigurements. There are special provisions for people with progressive or recurring conditions.