A condition from birth which makes an individual (usually a celebrity) have delusions of grandeur in one particular field (i.e. acting, for William Shatner). These people are not actually talented, they just feel they are due to Shatner's Disease. Also called The Worthless Gene. There is no known cure or treatment.
Why does Britney Spears actually think she can sing?
She was born with Shatner's Disease.