ACUTE AND CHRONIC DISEASES 233 



onset, and in their duration. As a rule, the more sudden and violent 

 the onset of a disease the shorter is its duration. Thus influenza, 

 common cold, pneumonia, diphtheria, smallpox, and many others, 

 develop and end quickly. After an illness of a few days the sufferer 

 dies or recovers at least he recovers from the actual disease, though 

 injuries (sequelae) inflicted by it may persist, for example deafness 

 from scarlet fever. Before his illness he is free from microbes, as 

 is proved by the fact that he does not infect his fellows ; during it 

 he swarms with them, for he is then very liable to convey infection ; 

 after recovery, since he is no longer infective, he is free from them 

 again. Something has banished the microbes. His body has 

 undergone a profound and remarkable change. From being a soil 

 in which the parasites were able to flourish and multiply, it has be- 

 come impossible as an abiding-place to them. He has acquired 

 immunity. He has developed, not under the stimulus of nutriment, 

 but under that of experience (use). This kind of immunity, which 

 follows recovery from disease, is obviously quite distinct from that 

 power of resisting infection which develops under the stimulus of 

 nutriment, and is termed inborn immunity, and is often seen in 

 Englishmen, for example in relation to tuberculosis. In the case 

 of some diseases, smallpox, measles and others, acquired immunity 

 is usually permanent, but in the case of others, for example common 

 cold, influenza, and diphtheria, it is generally temporary. 



390. In strong contrast to the acute diseases are such chronic 

 maladies as tuberculosis and leprosy. Here the onset of illness 

 is marked by no sudden symptoms of deep poisoning ; the complaint 

 pursues a prolonged indefinite course ; recovery, if it occurs, is as 

 gradual as the onset, and is commonly accompanied by exacerba- 

 tions and remissions of disease. The microbes are banished, not 

 because that particular bodily change which is termed * acquired 

 immunity ' supervenes, but because an improvement in general 

 health, due to better nutrition, or surroundings, or some such cause, 

 strengthens the individual, and with him his phagocytes, and enables 

 them to destroy their enemies. Often, in spite of apparent recovery, 

 the microbes may linger in the system for years ; and if the health 

 be lowered, the disease, uninfluenced by the past experience of the 

 individual, may be started afresh by surviving microbes, or by a 

 secondary infection from without, and so ultimately prove fatal. 

 Recovery from such chronic maladies of indefinite length, there- 

 fore, is precarious ; it affords no protection, no increase of resisting 

 power, no 'acquired immunity.' Unlike a man who has recovered 

 from smallpox, one who has experienced tuberculosis is always in 



