Visual - Associated medical conditions include myopia, color blindness, glaucoma and albinism
Hearing - Associated medical conditions include presbycusis, acoustic trauma, auditory processing disorder, and otosclerosis
Motor - Associated medical conditions include RSI, cerebral palsy, Parkinson's, and muscular dystrophy
Cognitive - Associated medical conditions include Down’s syndrome, autism, global developmental delay and dyslexia
source: http://www.netmagazine.com/features/simple-introduction-web-accessibility