The course of small-pox, whether distinct or confluent, may be divided into five stages, which are known under the names of incubation, invasion, eruption, suppuration, and desiccation. This division, founded on the most prominent symptoms that the disease presents, although it is an arbitrary one, still affords a facility in the study. The period of incubation comprises the interval of time that elapses from the infection to the beginning of the attack; its duration is from six to twenty days. It is not designated by any visible symptom, as the individual apparently continues in good health.