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,?
Disability
According to many definitions, a disability is an impairment that may be cognitive, developmental, intellectual, mental, physical, sensory, or some combination of these. Other definitions describe disability as the societal disadvantage arising from such impairments. Disability substantially affects a person's life activities and may be present from birth or occur during a person's lifetime.
Disability is a contested concept, with different meanings in different communities. It may be used to refer to physical or mental attributes that some institutions, particularly medicine,
is made, based on whether or not the applicant is able to perform work that used to be done in the past or that the applicant is trained to do and is likely to be able to perform and find employment in the disability. It is for that reason that two people with identical disabilities may have very differing outcomes in their applications.
The legal definition of a disability that qualifies an applicant for benefits is "inability to engage in any substantial gainful activity by reason of any medically determinable physical or mental impairment which