PATHOGENIC BACTERIA 117 



stand the distinction which is made between the two classes of 

 pathogenic bacteria. Bacteria may, for example, feed on cheese and 

 secrete a poison as one of the products of decomposition and then 

 die off. Their poisons will be present in the cheese. If a person 

 now eats a portion of the cheese he suffers from the effects of the 

 poison, although not a single live microbe of the species that pro- 

 duced that poison enters his body. This is different to the case 

 of the person who eats a portion of cheese which contains no poison, 

 but a few microbes of a kind that is able to grow and multiply inside 

 that person's body. In this case the poison is manufactured inside 

 the body, the microbe being the parasite and the person the host. 



2. BACTERIAL DISEASES OF THE ANIMAL KINGDOM. 



A large proportion of our food consists of substances upon which 

 bacteria as well as ourselves find nourishment. If a poison-secreting 

 microbe multiplies in food, and the latter is afterwards consumed, the 

 consumer suffers from the effects of the poison produced by the 

 microbe. In this way poisoning from eating decayed meat, cheese, 

 ice-cream, fish, etc., is caused, and the effect is generally rapid, though 

 if the bacteria are not alive when the bad food is eaten, recovery is 

 rapid so soon as the body throws off the poison, for there will be no 

 bacteria to produce more of it. In warm weather the danger from 

 this source is greater than in cold weather, because the temperature 

 favours a rapid multiplication of these organisms, and this is especially 

 so in the case of milk. The most important disease of this kind is the 

 Cholera infantum, which is common among infants who are nourished 

 entirely on cow's milk. We have a good instance of the other kind of 

 pathogenic bacteria in Bacillus typhosus, which is the cause of typhoid 

 fever. This microbe grows and multiplies in the intestines, and whilst 

 not invading the body generally, becomes localised in special glands 

 like the liver and spleen. The poison which is secreted is called typho- 

 toxine. In the case of diphtheria we have an instance of bacteria that 

 cannot be strictly included in either of the two classes of pathogenic 

 bacteria, for though Bacillus diphtheriae is found in the mouth and 

 throat of patients suffering from this disease, it is found only in the 

 false membrane of these parts. They do not enter into the deeper- 

 tissues of the body, though they grow and multiply in the false 

 membrane, secreting a poison which causes the disease. The bacillus 

 is composed of slender rods, about 3 ^ in length, and sometimes slightly 



