232 HUMAN DISEASES 



different diseases are unlike ; for example, the symptoms of the 

 poisoning occasioned by diphtheria are different in type from 

 those which occur in tetanus. Judging again by the symptoms, 

 the virulence of the toxins of some diseases is much greater than 

 that of others. Possibly there is not in nature a more deadly 

 poison than the tetanus toxin. The microbes, few in number and 

 located in a single small area, can produce only an exceedingly 

 minute quantity, yet the sufferer is thrown into violent convulsions, 

 and is often killed. On the other hand, even when large areas of 

 the body are affected by leprosy, the person afflicted feels compara- 

 tively well. In measles, smallpox, influenza, and the other acute 

 diseases, the infected person falls suddenly and violently ill. 

 Evidently he is suffering from intense poisoning ; but he may be in- 

 fected for months by tuberculosis before he is even aware of illness ; 

 and, even when he is dying, his symptoms are traceable to nutritive 

 changes, and to the irritation and destruction of tissues caused by 

 the microbes themselves, or to toxins and waste products produced 

 by the microbes of normally saprophytic species, which have 

 established themselves in the perishing areas of tissue, rather 

 than to toxins secreted into the surrounding medium by the bacilli 

 tuberculosis. Therefore, we may reasonably infer that, whereas 

 some species of microbes produce virulent toxins which pass into 

 the surrounding medium, the toxins of others are weak or are 

 retained within the microbes. 



388. This hypothesis is confirmed by another set of facts 

 In acute diseases, such as diphtheria and tetanus, though the 

 phagocytes crowd towards the infected area till the red and in- 

 flamed tissue surrounding it is full of them, yet, killed or paralysed 

 by the concentrated toxins, they do not ingest the microbes, at 

 any rate at first. If the sufferer dies they are unable to the end to 

 cope with the invaders ; but in cases of recovery they gradually 

 gain the upper hand, and in the later stages of disease the dis- 

 integrating microbes may be seen within them. On the other 

 hand, in such diseases as tuberculosis and leprosy, the phagocytes 

 ingest the microbes from the first. The struggle is then, in a way, 

 physical, not one conducted at long range like that fought out with 

 the microbes that produce virulent toxins. Victory depends, so to 

 speak, on personal prowess. If the microbes are victorious, they 

 spread and multiply, and the sufferer eventually dies ; if the 

 phagocytes are victorious, the microbes are exterminated and the 

 sufferer recovers. 



389. Microbic diseases differ greatly in the suddenness of their 



