This article is about an autoimmune neuromuscular disease Myasthenia Gravis.

This is what the National Institute of Neurological Disorders and Stroke says:

"Myasthenia gravis is a chronic autoimmune neuromuscular disease
characterized by varying degrees of weakness of the skeletal
(voluntary) muscles of the body. The hallmark of myasthenia gravis is
muscle weakness that increases during periods of activity and improves
after periods of rest. Muscles that control eye and eyelid movements,
facial expression, chewing, talking, and swallowing are often, but not
always, involved. The muscles that control breathing and neck and limb
movements may also be affected."

The reason for the muscular failure is due to antibodies produced by the body's own immune
system which block, alter, or destroy the receptors for acetylcholine essential for the travel of nerve impulse.

Diagnosis can be confirmed by special blood test that can
detect the presence of immune molecules or acetylcholine receptor antibodies.