*Duration of Immunity.*--Sometimes a single vaccination gives immunity for life. Usually, however, susceptibility returns at latest seven to ten years after vaccination, and the second vaccination may give immunity for the rest of the lifetime, or susceptibility may return again and again. Failure of active, properly inserted virus shows only that the person so vaccinated is at that time immune, but conveys absolutely no information about the condition a few months later. The appearance of the scar is not a trustworthy guide as to immunity. Susceptibility to vaccination returns frequently within one year, and has returned in three months from the time of a successful vaccination. Susceptibility to smallpox probably returns, as a rule, later than susceptibility to vaccinia. It is rare that a case even of varioloid occurs within five years of a successful vaccination. *Conclusions.*--Every child should be vaccinated at the time of election during the first year of life, and should be revaccinated before beginning school-life with its possibility of exposure. Every person, no matter at what age, should be vaccinated at a time of possible exposure to smallpox unless he has been successfully vaccinated within three months. [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA ERYTHEMATOSA. (First day of eruption).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA PAPULOSA. (Second day).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA HEMORRHAGICA. (Second day--Fourth day).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA VESICULOSA. (Fourth day).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA VESICULOSA. (Third day--Fifth day--Sixth day).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA SEMI-CONFLUENS. (Fifth day--Sixth day).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA CONFLUENS. (Seventh day--Eighth day).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA PUSTULOSA. (Ninth day).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA DISCRETA. (Ninth day).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA PUSTULOSA. (Tenth day).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA PUSTULOSA. (Ninth day--Tenth day--Eleventh day).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA PUSTULOSA ET CRUSTOSA. (Tenth day--Twelfth day).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA CRUSTOSA. (Eighteenth day).] [Illustration: Copyright, 1902, by G. H. Fox. VARIOLA DESICCATA ET SQUAMOSA. (Twentieth day).] [Illustration: Copyright, 1902, by G. H. Fox. 1. PIGMENTATION AFTER VARIOLA. (30th day). 2. VERRUCOUS SCARS. (25th day). 3. CONFLUENT PITTING. (35th day).] [Illustration: Copyright, 1902, by G. H. Fox. 1. VACCINIA. (4th day). 2. VACCINIA. (8th day). 3. PRIMARY VACCINATION. (8th day). 4. VACCINIA. (8th day). 5. VACCINATION ULCER. 6. VARICELLA. (3d day).]