44 BACTERIOLOGY 



number of these antibodies, as they are called, but many others 

 it has to elaborate for itself as the result of experience. The 

 function of these antibodies is probably, as in the case of 

 phagocytosis, merely a modification of the natural process of 

 food-digestion ; and the infant has to learn to become immune, 

 not only to bacteria, etc., but to the various kinds of food upon 

 which it must acquire the capacity of living. A certain amount 

 of such immunity is transferred by the mother to her infant 

 before birth, and also by her milk, but after birth the child's 

 tissues have to learn to fight their own battles against the 

 bacteria which speedily gain access to its skin and alimentary 

 tract. Normally this process is a gradual one ; the tissues learn 

 their lesson and the invaders are kept out ; but, if the child is 

 feeble, if disease-producing organisms are present in its food, 

 and especially if that food be abnormal, and if other factors are 

 unfavourable, such as absence of light and air, etc., its powers of 

 resistance are unequal to the strain, the bacteria gain entrance, 

 and the child becomes ill, i.e. the fight becomes so intense that 

 the symptoms we are accustomed to associate with illness mani- 

 fest themselves symptoms which are often rather a sign that 

 the fight against the invader has become more severe, than that 

 the illness has just commenced. In the case of many of the 

 specific infective fevers of childhood, e.g.. Scarlet Fever, Measles, 

 Whooping-Cough, etc., once the struggle is successfully over, a 

 more or less lasting and complete immunity against that special 

 disease is established. Similarly, one attack of Smallpox pro- 

 tects against another attack. Unfortunately, however, there 

 are some diseases after which the period of immunity is short 

 or even absent. Influenza, Pneumonia, and many forms of 

 Common Cold are frequently followed by relapses and recur- 

 rences. Some diseases also, by their weakening action upon 

 the body, render the tissues more liable to invasion by other 

 organisms, and it is for this reason that Measles and Whooping- 

 Cough, Pneumonia and Influenza, are so often followed by 

 broncho-pneumonia, or by tuberculosis, these diseases preparing 

 the way, and rendering more easy the entrance and multiplica- 

 tion of strepto- and pneumococci or of the tubercle bacillus. 



Immunity, like pathogenicity or the power of the attacking 

 organism to produce disease, is relative and not absolute. If 

 an organism capable of producing disease passes through a 

 series of relatively susceptible animals, it usually becomes more 

 and more virulent, i.e. its power of attacking and causing 

 disease in such animals is increased. This is one important 

 factor in the production of some epidemics such as influenza, 

 where the organism passes directly from host to host, for in- 

 stance in the fine spray produced by coughing or sneezing. 



