Inability, disability are sometimes confused because of their verbal likeness. Although both denote a lack of ability to perform a given act or to follow a given trade or profession, they are otherwise clearly distinguished.
Inability implies lack of power to perform; it may suggest mental deficiency or tempermental unfitness, but more often it suggests a limiting factor (as lack of means, lack of health, or lack of training).
Disability implies the loss or the deprivation of such power (as by accident, illness, or disqualification); the term is applicable not only to the resulting inability but to whatever it is that makes one unable to do a certain thing or hold a certain office or position.