*Grippe.*--An attack of grippe may simulate the early symptoms of smallpox very closely. The onset may be sudden, the muscular pains severe, the pyrexia decided, the general prostration as marked as in smallpox. In grippe, however, the muscular pains are, as a rule, more general than in smallpox, there is rarely profuse sweating, and symptoms referable to the respiratory tract soon develop, if indeed they are not present from the beginning. *Rheumatism.*--The severe lumbar and sacral pains of smallpox have been mistaken for rheumatism, but such an error can be made only where the use of the clinical thermometer is unknown. A febrile movement in lumbago is absent or but slight, while in smallpox the pyrexia is usually pronounced. *Typhoid and Typhus.*--Typhoid and typhus fevers have at times been confounded with smallpox. But errors of this kind can be made only where the history of the case is completely ignored. In typhus, it is true, the eruption, petechial and almost papular in character, may suggest hemorrhagic smallpox; but the eruption of typhus rarely appears before the fourth or fifth day of the illness and is located chiefly on the trunk, sparing the face. The rash of malignant smallpox develops usually on the third or even the second day of the illness and is not limited to the trunk. Upon the appearance of the rash in a typical case of smallpox the febrile diseases with which it is most frequently confounded are measles and varicella. It is interesting to note that until the time of Sydenham, in the latter part of the seventeenth century, measles and smallpox were regarded as manifestations of the same disease, and that the Vienna school of dermatologists, even to this day, insists on the etiological unity of variola and varicella. *Measles.*--As a matter of fact the early papular eruption of measles bears a considerable resemblance to the first stage of the eruption of smallpox. In both the eruption is noted first in the face. In smallpox, however, the papules have a firm, “shotty” feeling on palpation, while in measles they are smooth and velvety to the touch. In measles the eruption, viewed at a little distance, seems to present a distinctly corymbose or crescentic grouping, an arrangement which is absent in smallpox. The eruption of smallpox appears at the end of the third day, that of measles on the fourth day. The temperature in smallpox undergoes a rapid defervescence upon the appearance of the rash, while in measles it continues to rise after the eruption appears. The pronounced pain in the back is absent in measles, while the very marked catarrhal symptoms, coryza, conjunctivitis, etc., are lacking in smallpox. The subsequent course of the eruption will leave no room for doubt, since within twenty-four hours the papules of smallpox will have developed into characteristic vesicles.