The disease has a world-wide distribution, all races are equally involved. The disease can manifest at any age from newborn to old age. It can rarely be congenital. Though no age group is immune from the disease it is commonly seen between 20 and 60 years of age. The incidence of typical myasthenia falls sharply in and after seventh decade when atypical myasthenia becomes more common, which may involve plain muscles resulting in paralysis of iris and ciliary body. The disease is seen more in women as compared to men in a ratio of 3:2. A pregnant woman may pass the antibodies responsible for the disease to the unborn child who may show the features of the disease soon after birth. Though myasthenia is not a true hereditary disorder, family history is positive in as much as 5% patients. The disease is chronic disease generally progressive or intermittent. In some patients, the disease is self limiting and the clinical features disappear without treatment. Rarely the disease may pass into crisis. The chronic cases may become fixed, i.e. not responding to treatment, which should alert the physician to think of other conditions as well, i.e. chronic progressive external ophthalmoplegia.