500 SMALLPOX AND VACCINATION. 



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 attack outlasts its power wholly to 

 ward it off. Revaccination restores protection, but this 

 operation must be from time to time repeated. Vaccina- 

 tion 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 

 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 intro- 

 duced 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 



