The Devil's Dictionary
An interior convulsion, producing a distortion of the features and accompanied by inarticulate noises. It is infectious and, though intermittent, incurable. Liability to attacks of laughter is one of the characteristics distinguishing man from the animals— these being not only inaccessible to the provocation of his example, but impregnable to the microbes having original jurisdiction in bestowal of the disease.
Whether laughter could be imparted to animals by inoculation from the human patient is a question that has not been answered by experimentation. Dr. Meir Witchell holds that the infectious character of laughter is due to the instantaneous fermentation of sputa diffused in a spray. From this peculiarity he names the disorder Convulsio spargens.