r6 BIOLOGICAL LECTURES. 



exciting cause which develops an attack of pneumonia or 

 influenza or tuberculosis in an individual having a predisposi- 

 tion to these diseases, — the specific cause being present; or 

 indigestible food in the priniae viae may be the exciting cause 

 of an attack of cholera ; or the irritation resulting from breath- 

 ing an atmosphere loaded with dust may give rise to tubercular 

 infection. Again, bruising of the tissues may give rise to an 

 abscess or a carbuncle in an individual whose vital resisting 

 power is below par, the specific agents of such local infections 

 (the pus cocci) being widely distributed and frequently found 

 on the surface of the body and upon exposed mucous mem- 

 branes in healthy individuals. 



A non-fatal attack of an infectious malady, as a rule, is 

 followed by a relative immunity of longer or shorter duration. 

 In smallpox, scarlet fever, measles, yellow fever, typhoid fever, 

 and certain other diseases of man one attack usually protects 

 during the life of the individual; but exceptions to this rule are 

 not rare, especially in the case of smallpox. In pneumonia, 

 diphtheria, influenza, and cholera second attacks are frequent ; 

 and while a relative degree of immunity is no doubt acquired 

 as a result of an attack, this is of brief duration. 



The production of immunity by protective inoculations was 

 for a long time limited to a single disease, — smallpox. 

 Inoculations with virus, obtained from a pustule on a smallpox 

 patient, were extensively practised before the discovery of 

 vaccination by Jenner. These inoculations gave rise to a mild 

 attack of the disease, followed by immunity, which was appar- 

 ently as complete as that following a more severe attack con- 

 tracted in the usual way. This method seems to have been 

 practised by eastern nations long before it was introduced into 

 Europe. *It was extensively employed in Turkey early in the 

 eighteenth century, and was introduced into England through 

 the influence of Lady Mary Wortley Montagu. No doubt the 

 mortality from smallpox was greatly diminished by these 

 inoculations ; but they were attended by the disadvantage that 

 the disease was propagated by them, inasmuch as inoculated 

 individuals became a source of infection for others. Inocula- 

 tion was still practised in England for some time after the 



