When an injury disables someone, it leaves the person seriously hurt, probably for the rest of their life. If a car accident disables you, you are permanently limited in some physical way.
Illness, as well as injury, can disable people: for example, diabetes disables some sufferers by leaving them blind or weakening their hearts. A completely different way to disable is to make something unusable or unable to work in a certain way. if you disable your smoke detector while you cook nachos in your smoky oven, it won't work until you reconnect the batteries.