Variola, or smallpox, is an acute, contagious disease, characterized by an eruption upon both the skin and mucous membrane, with constitutional symptoms of greater or less severity. The eruption presents successively a macular, papular, vesicular, and pustular stage, the pustules finally drying into crusts, which fall and leave the skin temporarily discolored. Where ulceration has occurred it is permanently scarred or pitted. The lesions of the mucous membrane appear upon those parts more or less exposed to the air,--the mouth and eyes, for example,--but in exceptional cases they may be found throughout the entire intestinal tract, and in the uterus and bladder. These lesions do not run a course similar to those observed upon the skin, but appear as red macules, which rapidly change into ulcerations, covered with a whitish pellicle. The ulcers are imbedded in the substance of the mucous membrane and are not as superficial as in cancrum oris. The constitutional symptoms are most prominent during the periods of invasion and pustulation. There are various clinical forms of smallpox, which may be conveniently described as (1) discrete, (2) confluent, and (3) hemorrhagic, or malignant; and then, according to intensity, as (_a_) very mild, (_b_) mild, and (_c_) severe. The few purpuric spots seen in the severe discrete and the confluent forms are not of great significance, as they are generally due to a peculiar diathesis, and as a rule the patient recovers. The malignant form is almost invariably fatal. The term discrete implies that the lesions are separate and distinct, not coalescent. If the lesions coalesce and form patches of various shapes and sizes, the eruption is called confluent. For the purpose of differentiating the various forms above mentioned, it is convenient to first trace a normal, unmodified case of smallpox from the initial symptoms to recovery, and then to consider the severe forms, and finally the rare and obscure forms of the disease. *Period of Incubation.*--This extends from the date of exposure to the occurrence of clinical symptoms, a period usually lasting from twelve to fourteen days. *Period of Invasion.*--The disease is usually ushered in by fever, with a distinct chill or chilly sensations, headache, neuralgia, and a general malaise. Frequently the first symptom is a distressing backache. This is located in the lumbar region, but it may be as high up as the lower angle of the scapula, or it may be sacral and extend down into the thighs. The backache is an important symptom when present, but it is not always on hand to help one out in the diagnosis. The backache of smallpox is not peculiar or distinctive, but it is its severity which attracts attention. The headache is usually frontal and is an ache that is constant in character. The neuralgia is about the orbits, but may be facial, and is of a lancinating character.