606 SMALLPOX AND VACCINATION 



those vaccinated as infants escape more or less till after the 

 thirteenth to the fifteenth years. Eevaccination is therefore 

 necessary if immunity is to continue ; and where this is done 

 in any population, smallpox becomes a rare disease, and 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 opera- 

 tion must be from time to time repeated. Vaccination is 

 beneficial according to the thoroughness with which it is per- 

 formed. 



The Relationship of Smallpox (Variola) to Cowpox 

 (Vaccinia). — This is a question regarding which great contro- 

 versy 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. In the parallel condition in the guinea-pig, 

 however, Camus has produced a general eruption by the intra- 

 venous injection of calf lymph. The loss of infectiveness by 

 transmission through the body of a relatively insusceptible 

 animal is a condition which is familiar in other diseases, and the 

 uniformity of the type of the affection resulting from vaccination 

 with calf lymph finds a parallel in hydrophobia, where, after 

 passage through a series of monkeys, a virus of attenuated but 

 constant virulence can be obtained. In considering the relation- 

 ships of cowpox and smallpox, the immunity which the virus 

 of calf lymph confers against human smallpox is an important 

 though subsidiary point. It has been found that monkeys 

 treated with vaccine lymph become after a few days immune 

 against infection with variola. The significance of such an 



