RELATIONSHIP OF SMALLPOX TO COWPOX 605 



the unvaccinated individuals who have contracted the disease. 

 While vaccination is undoubtedly efficacious in protecting against 

 smallpox, Jenner was wrong in supposing that a vaccination in 

 infancy afforded protection for more than a certain number of 

 years thereafter. It has been noted in smallpox epidemics 

 which have occurred since the introduction of vaccination, that 

 whereas young unprotected subjects readily contract the disease, 

 those vaccinated as infants escape more or less till after the 

 thirteenth to the fifteenth years. It has become, therefore, more 

 and more evident that revaccination is necessary if immunity is 

 to continue ; and where this is done in any population, smallpox 

 becomes a rare disease, as has happened in the German army, 

 where the mortality is practically nil. The whole question of 

 the efficacy of vaccination was investigated in this country in 

 1896 by a Royal Commission, whose general conclusions were 

 as follows : Vaccination diminishes the liability to attack by 

 smallpox, and when the latter does occur, the disease is milder 

 and less fatal. Protection against attack is greatest during 

 nine or ten years after vaccination. It is still efficacious for a 

 further period of five years, and possibly never wholly ceases. 

 The power of vaccination to modify an attack outlasts its power 

 wholly to ward it off. Revaccination restores protection, but 

 this operation must be from time to time repeated. Vaccination 

 is beneficial according to the thoroughness with which it is 

 performed. 



The Relationship of Smallpox (Variola) to Cowpox 

 (Vaccinia). This is the question regarding which, since the 

 introduction of vaccination, the greatest controversy has taken 

 place ; a subsidiary point has been the inter-relationships within 

 the group of animal diseases which includes cowpox, horsepox, 

 sheep-pox, and cattle-plague. With reference to smallpox and 

 cowpox the problem has been, Are they identical or not 1 There 

 is no doubt that cowpox can be communicated to man, in whom 

 it produces the eruption limited to the point of inoculation, and 

 the slight general symptoms which vaccination with calf lymph 

 has made familiar. Apparently against the view that cowpox 

 is a modified smallpox are the facts that it never reproduces in 

 man a general eruption, and that the local eruption is only 

 infectious when matter from it is introduced into an abrasion. 

 The loss of infectiveness by transmission through the body of a 

 relatively insusceptible animal is a condition of which we have 

 already seen many instances in other diseases, and the uniformity 

 of th<; type of the affection resulting from vaccination with calf 

 lymph finds a parallel in such a disease as hydrophobia, where, 



