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The fact is thus fully established that the protection afforded by successful vaccination varies in time with different cases, and that the liability to varioloid is greatest between the ages of 15 and 25 years.

Statistics further show that about one half of those who have had the vaccine disease as a result of vaccination, are liable to a modified form of small pox called varioloid, approaching more or less in violence to malIgnant types of the disease.

First, if there has been exposure, there is time for vaccination to be performed, and to develop the vaccine disease before the small pox shall appear, and to modify the latter into the milder type of varioloid.

With the present light upon the subject, it would seem to be an instance of the origination anew of a malignant type of varioloid disease.

The identity of varioloid with variola is abundantly shown—first, by the occurrence of intermediate forms of every grade, from the mildest evidence of variolous poisoning to typically developed cases of variola vera; second, by the fact that patients affected with varioloid are capable of transmitting variola to the unprotected; third, by the anatomico-pathological fact that the structure of the pock, when it appears, is the same in all.