362 BIOLOGY: GENERAL AND MEDICAL 



the sign of the cross; but this has a very ill effect, all 

 of these wounds leaving little scars, and is not done by 

 those that are not superstitious, who choose to have them 

 in the legs or that part of the arm that is concealed. 

 The children or young patients play together all the 

 rest of the day and are in perfect health to the eighth. 

 Then the fever begins to seize them, and they keep their 

 beds two days, very seldom three. They have very 

 rarely above twenty or thirty in their faces, which never 

 mark; and in eight days' time they are as well as before 

 their illness. Where they were wounded there remain 

 running sores during the distemper, which, I don't doubt, 

 is a great relief to it. Every year thousands undergo 

 this operation; and the French Ambassador says pleas- 

 antly that they take the small-pox here by way of 

 diversion as they take the waters in other countries. 

 There is no example of anyone that has died in it; and 

 you may believe I am very well satisfied of the safety 

 of this experiment, since I intend to try it on my dear 

 little son." 



The next experimental application of the principle of 

 preventing an infectious disease by giving an attack of a 

 disease was made by Edward Jenner, an English physi- 

 cians and naturalist, once a pupil of John Hunter, in 

 whose family he lived. For some time Jenner had been 

 engaged in the study of small-pox, cow-pox, and swine- 

 pox, and the development of the latter two diseases when 

 communicated to man. He at first made the mistake 

 of believing cow-pox to be caused by the contagion of a 

 peculiar hoof disease of horses known as " grease." He 

 first suggested that an attack of cow-pox would prevent 

 small-pox, and that it might be experimentally em- 

 ployed for that purpose, in a conversation with William 

 Hunter as early as 1770. A German schoolmaster, 

 Nicholas Plett, had already held the same idea and had 

 made certain proofs, but the matter had gone no further. 

 Jenner inoculated his own son with swine-pox and 

 later found him immune against small-pox. Fearing 



