Disability is inability to do what one is capable of,by own definition though. Disability is an impairment that may be cognitive, developmental, intellectual, mental, physical, sensory.
When is one said to have disabilities?
It has different contextual meaning 
So what are the contextual meanings you alluded?
refer to limitations imposed on people by the constraints of an ableist society
Are there laws that protects people with disabilities?
Interestingly,it arose out of disability activism in the United States and the United Kingdom in the 1970s, which challenged how the medical concept of disability dominated .
So what could be the major cause of disabilities, is it medically caused,?
contested concept, with different meanings for different communities.