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>I hope I have spelled this right.  I have seen this referred to a number
>of times in the last couple of weeks and wondering what it is.  Anyone
>care to put it into laymens terms.

The following is from The Stein and Day International Medical
Encyclopedia 1971 (originally published in England as Pears Medical
Encyclopedia, Illustrated):

Myasthenia Gravis: a rare condition in which the muscles become
fatigued abnormally quickly. It is found most often in young adults.

Cause: at present not fully understood. There is a defect of function
at the junction between the nerve and muscle fibers, caused by a lack
of acetylcholine, the subsatnce necessary for transmission of nerve
impulses.  This is thought to be due in some way to disorder of the
thymus gland in the upper part of the chest, possibly through an immune
reaction.

Symptoms: at first weakness of the muscles supplied by the cranial
nerves, so that the upper eyelid droops, there is double vision, and
difficulty in speech, swallowing and breathing. Later any muscle may
become involved.

Treatment: edrophonium chloride is injected intravenously to make the
diagnosis, for it has a dramatic action on the disease. It is not,
however, used in treatment because the action only lasts a short time.
Longer-acting drugs are neostigmine and pyridostigmine, which are taken
in doses varying according to the patient's condition. The thymus gland
is removed surgically in many cases of which about one-third may be
expected to show a marked improvement; but the operation is not usually
carried out in patients over 50.