It depends on the type of disability and the severity.
For example, if you are quadriplegic, then playing games is simply out of the question. If you have a severe case of carpal tunnel syndrome in one hand then it is possible to play some types of games with just your good hand.
there are lots of adaptations available for less physically capable gamers.
there are basic controller headbands...(admittedly more hype than use) but they do have basic functionality... there are plenty of different kinds of controllers out there so you dont have to stick with mouse and keyboard. everything from thumb controllers like the wii remote. to track balls, gamepads and wheels. there are many ways a differently able person can get into gaming... even if there vision isnt the best they can use the ease of access center in windows to help increase the screeen size of fonts and images.
i myself am registered disabled due to an accident i had a long time ago. im left with constant pain and take ridiculously strong pain killers 4 times a day. i cant walk 50 yards without being in severe discomfort. but i can sit long enough to have a round of bf3... 1 of the reasons i got my current keyboard was because of the way i sit. i tend to favor my left butt cheek and rest a lot of my weight on my left elbow. but i also type with my left hand which often would start falling asleep in game... giving me pins and needles. i swapped the dome keys for mechanical and i no longer have that issue. its a simple swap but the keys are lighter to press so im not burning so much oxygen in my muscles so my are doesn't fall a sleep. a simple but effective solution to a problem i had with gaming...