460 SMALLPOX AND VACCINATION. 



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 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 ? 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 cow- 

 pox 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 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 con- 

 stant 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 im- 

 portant though subsidiary point ; for at present it is question- 

 able whether there are any well-authenticated instances of 

 one disease having the capacity of conferring immunity 

 against another. The most difficult question in this con- 

 nection is what happens when inoculations of smallpox 

 matter are made on cattle. Chauveau denies that in such 



