SMALLPOX AND VACCINATION. 503 



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 ques- 

 tion of the efficacy of vaccination was investigated in Great Britain in 

 1896 by a Royal Commission, whose general conclusions were as fol- 

 lows. Vaccination diminishes the liability to attack by smallpox, and 

 when the latter does occur, the disease is milder and less fatal. Protec- 

 tion 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 attack outlasts its 

 power wholly to ward it off. Revaccination restores protection, but this 

 operation must be from time to time repeated. Vaccination is benefi- 

 cial 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 vaccina- 

 tion, 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 refer- 

 ence to smallpox and cowpox the problem has been, Are they identical 

 or not ? 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 erup- 

 tion, 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 the type of the affection resulting from vaccination with 

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

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

 constant virulence can be obtained. We have seen that there are good 

 grounds for believing that the virus of calf lymph confers immunity 

 against human smallpox. In considering the relationships of cowpox 

 and smallpox, this is an important though subsidiary point ; for at 

 present it is questionable whether there are any well-authenticated 

 instances of one disease having the capacity of conferring immunity 

 against another. The most difficult question in this connection is what 

 happens when inoculations of smallpox matter are made on cattle. 

 Chauveau denies that in such circumstances cowpox -is obtained. He, 

 however, only experimented on adult cows. The transformation has 

 been accomplished by many observers, including, in Britain, Simp- 

 son, Klein, Hime, and Copeman. The general result of these experi- 



