THEORIES EXPLAINING ACQUIRED IMMUNITY. 121 



anxiety, fear and fright, and improper diet, all predispose to 

 infection by lessening immunity. 



Noxious gases : The inhalation of noxious gases also tends 

 to diminish the condition of immunity. Since the perfection 

 of the sewerage system, infectious diseases are not so often the 

 result of inhalation of sewer gases. Formerly many diseases 

 were supposed to have their origin in the inhalation of 

 sewer gas, especially diphtheria and scarlet fever ; but since 

 their infectious nature has been established, it must be evi- 

 dent that these gases are only operative in so far as they 

 lessen the bodily resistance, or immunity, to these infectious 

 diseases. Modern sanitation has certainly been effective in 

 reducing the number of cases of the diseases mentioned, and 

 we must, therefore, accept the statement that noxious gases 

 play some part in their production. 



Drugs, etc. : Persons addicted to the use of drugs or alcohol 

 are much more liable to infection than total abstainers. In 

 such individuals the disease usually manifests itself in its 

 worst form. 



Trauma and operations : These invariably reduce immunity 

 by creating conditions most conducive to the development of 

 bacteria. 



Other diseases : Any chronic disease reduces the vitality of 

 the individual, so that he is extremely susceptible to infection. 

 His immune power is all used up, and he succumbs to the 

 terminal infections, which he would escape were he in posses- 

 sion of his full powers of resistance. 



Mixed infections : The body appears to be unable to take 

 care of more than one infection at a time. Its energies are 

 directed toward one point of attack, and dissolution follows 

 as the result of the other infection. Persons who have sur- 

 vived an attack of typhoid or influenza succumb rapidly to a 

 complicating pneumonia. The two occurring together is more 

 to be dreaded than either one alone. 



THEORIES EXPLAINING ACQUIRED IMMUNITY : Exhaustion 

 theory of Pasteur and Klebs: In 1880 Pasteur and Klebs 

 suggested that immunity was due to the exhaustion in the 

 body of some substance necessary to the development of 

 bacteria. During the first infection the germs consumed this 



