*Zoster.*--Zoster is, as a rule, readily distinguished by the definite grouping of the lesions in the tract supplied by one or more nerves, its asymmetrical distribution, and the more or less severe neuralgic pain that precedes or accompanies the eruption. It must be remembered, however, that in zoster, in addition to the typical grouped lesions, there are occasionally seen a few isolated vesico-pustules scattered promiscuously over the entire surface; and the difficulty of diagnosis may be increased by the occurrence of a moderate temperature movement. In these cases, to which attention was first called by Teneson, the history of the case, the presence of characteristic herpetic groups, and the evolution and course of the individual lesions will suffice to clear the diagnosis. *Drug Eruptions.*--The ingestion of bromides, iodides, and quinine is sometimes followed by an eruption which may create some confusion in diagnosis. In general the drug eruptions may be distinguished by the absence of fever and of the subjective symptoms of smallpox. The bromide and the iodide acne never occur on the palms and soles, where there are no sebaceous glands, and the lesions lack the evolution and course of the variolous eruption. The erythematous and purpuric eruption of quinine may be confused with the hemorrhagic form of smallpox; but here, too, the history of the course of the illness and the absence of fever will obviate the difficulty. *Syphilis.*--Of all the diseases of the skin it is the pustular syphilide which most resembles the lesions of smallpox. Dermatologists and experts in variola are agreed that the pustular syphilide may be absolutely indistinguishable from smallpox so far as the appearance and distribution of the lesions is concerned. Furthermore, the pustular syphilide is frequently accompanied by a decided febrile movement. The differential diagnosis can be made in these cases only by the closest inquiry into the history of the case and by careful observation of the course of the disease. The characteristic history of an acute illness of short duration followed by a remission on the appearance of the eruption will of course be wanting in syphilis. The syphilitic eruption is more sluggish in its evolution as well as in the course of its subsequent changes; and though there may be lesions of syphilis on the mucous membrane of the mouth, they will lack the characteristic appearance of the vesicles and pustules of smallpox in this region. The palms and soles are not apt to show any lesions in this form of syphilis; and finally some other forms of syphilitic manifestation are very often present in this polymorphic disease to give the clue to the real nature of the eruption.