The recognition of a case of smallpox may be simple, difficult, or even impossible, depending on the case and on the stage of the disease. In general the disease is readily recognized when the case is typical and the eruption has reached the vesicular or pustular stage. The diagnosis is difficult in atypical and complicated cases. It is impossible with any degree of positiveness in most cases in the pre-eruption period,--the stage of invasion. The initial symptoms of smallpox resemble the first symptoms of so many infectious fevers that it is only through a consideration of the prevalence of an epidemic and the opportunities for infection in a given case that the physician may be put on his guard. It is important in this connection to notice whether the patient has been successfully vaccinated within a recent period. The physician who during the prevalence of an epidemic finds an unvaccinated subject suffering from a febrile disease of acute onset, with severe lumbar and dorsal pains, may, in the absence of definite symptoms pointing to some other disease, suspect smallpox; but a positive diagnosis at this stage is, of course, impossible. *Prodromal Rashes.*--The occurrence of the prodromal rashes, the roseola variolosa,--a more or less diffuse scarlatiniform, morbillic, or urticarial rash which may appear on the second day of the fever,--has a certain diagnostic value; but this roseola occurs in only a small percentage of the cases, and, unfortunately, sometimes appears in other acute toxæmic conditions,--typhoid, for instance. The scarlatiniform rash may lead to a diagnosis of scarlet fever and the morbillic roseola be mistaken for measles; but these diseases would be excluded by the absence of the angina and the strawberry tongue of scarlatina in the one case and of the catarrhal symptoms of measles in the other, aside from other considerations. The appearance of the eruption on the second day of scarlatina is followed by a marked defervescence, while the scarlet rash of smallpox is not accompanied by any change in the temperature curve. The eruption in measles occurs on the fourth day of the illness, a circumstance which alone suffices to differentiate it from the morbilliform roseola of smallpox. The characteristic and pathognomonic “Koplik spots” on the buccal mucous membrane in measles are, of course, absent in smallpox. Furthermore, these prodromal eruptions of variola are of extremely evanescent character and usually disappear within eight or ten hours. Of somewhat greater diagnostic value in this stage is the appearance of small hemorrhages, or petechiæ, varying in size from a pin’s head to a pea, in the brachial and crural triangles of Simon. This form of prodromal eruption, however, is extremely rare, and, it may be added, is of grave prognostic significance, as it is usually the precursor of hemorrhagic smallpox.