CHAPTER XIV [454] 



IMMUNITY ACQUIRED BY NATURAL MEANS 



imunity acquired after recovery from infective diseases.— Imnrnnity acquired in 

 malaria. — Humoral properties of convalescents from typhoid fever.— Preventive 

 power of the blood of persons who have recovered from Asiatic cholera. — 

 Antitoxic power of the blood of persons who have recovered from diphtheria. 



Immunity acquired by heredity. — Absence of hereditary immunity properly so-called. 

 — Immunity conferred by the maternal blood and by the yolk. 



Immunity conferred by the milk of the mother. 



It has long been known that an attack of one of many of the in- 

 fective diseases brings about a refractory condition of the organism 

 against that disease, a condition which persists for many years, and 

 may even endure for life. Even before the microbiological era of 

 medical science had arrived it had been fully established that a 

 person who had recovered from small-pox might come in contact 

 with and nurse small-pox patients without risk of contracting a 

 second attack of the disease. The same thing has been observed 

 purely empirically in several other infective diseases, such as 

 whooping-cough, typhoid fever, scarlatina, mumps, etc. On the 

 other hand it has been shown that certain infective diseases, such 

 as fibrinous pneumonia, erysipelas, recurrent fever, and influenza, 

 do not leave behind them the slightest trace of an immunity. It 

 has often been observed, indeed, that after a first attack of any of 

 these diseases there is a marked susceptibility to a second attack. 

 Between these two extremes come the infections which are followed 

 merely by a refractory condition of shorter duration than that which 

 follows the diseases of the first group. The first of this intermediate 

 group is measles, which gives rise to a relatively long immunity, then 

 come in order bubonic plague, anthrax, cholera, etc. 



It should be stated that the first attack of any of the infective 

 diseases causes modifications more or less permanent in the organism, 

 B. • 28 



