CHICKENPDX or varicella, a specific contagious disease characterized by an eruption of vesicles in the skin. The disease usually occurs in epidemics, and the patients are generally be tween two and six years old. The incubation period is from ten to 15 days; there are practically no prodromal symptoms, though slight fever for some 24 hours may precede the eruption. A number of raised red papules appear on the back or chest ; in from 12 to 24 hours these develop into tense vesicles filled with a clear fluid, which in another 36 hours or so becomes opalescent. During the fourth day these vesicles shrivel up, and the scabs fall off, leaving as a rule no scar. Fresh spots appear during the first three days, so that at the end of that time they can be seen in all stages of growth and decay. The eruption is most marked on the chest, but it also occurs on the face and limbs, and on the mucous membrane of the mouth and palate. The temperature begins to fall after the appearance of the rash, but may some times persist slightly after the disappearance of all symp toms. It rarely rises above Io2° F. The disease runs a very favourable course in most cases, and after-effects are rare. One attack does not give immunity. The diet should be light, and the pa tient should be prevented from scratching the spots, which would lead to ulceration and scarring. After the first few days there is no necessity to confine the patient to bcd. In the large ma jority of cases, it is easy to distinguish the disease from small pox, but sometimes it is very difficult. The chief points in the differential diagnosis are as follows : (I) In chickenpox the rash is chiefly on the trunk, and less on the limbs and face; (2) some of the vesicles are oval, whereas in smallpox they are always hemispherical. They have not at the outset the hard shotty feeling of the more virulent disease; (3) the vesicles attain their full growth within 12 to 24 hours; (4) the rash appears "in crops" ; (5) there is no prodromal period. No doubt the disease is caused by a but so far this has not been determined with certainty. The tendency is to ascribe it to a filter-passing virus (q.v.).