534 MICROBIOLOGY OF THE DISEASES OF MAN AND ANIMALS. 



disease may be induced in men by the ingestion of Bad. pestis, but in 

 some susceptible animals, such as rats, it is rapidly acquired in a very 

 fatal form when the bacteria enter the intestines. 



RESISTANCE. -This factor is one of the prominent ones which modify 

 the results of an infection. It is a familiar fact that two or more in- 

 dividuals may receive the same microorganism, as for example B. typho- 

 sus, one will not become infected or have a very mild form of the disease, 

 while the other will have the severest and most fatal form of typhoid 

 fever known. Again, the age of the individual infected is important 

 in determining the resistance. The adult resists infection such as diph- 

 theria, scarlet fever, and measles more than the child. The very young 

 child resists pneumonia and tuberculosis more than the adult. The 

 resistance depends on the presence in the animal body of natural or 

 acquired anti-bodies. It is therefore obvious that the higher the resist- 

 ance of the individual infected, the less severe will be the results of the 

 infection of that individual. 



THE EXACT CAUSE OP INFECTIONS. 



We are familiar with the fact that all of our infectious diseases are due 

 to microorganisms or- viruses of some form or other. The causal agents 

 of the majority of these diseases are known but in the case of those that are 

 not known there is reasonable certainty as to the types of the infecting 

 agents. They will be briefly considered as follows: 



SOLUBLE TOXINS. There are some pathogenic bacteria which secrete 

 through their cell walls poisons which diffuse into the surrounding media. 

 To these poisons or soluble toxins the disease symptoms are due. Bad. 

 diphtheria, of diphtheria, B. tetani, of lock-jaw or tetanus, Bad. dysen- 

 teria:, of bacillary dysentery, B. botulinus, of meat poisoning, and Ps. 

 pyocyanea, the causal organism of blue-green pus, are the most import- 

 ant bacteria of this character. Some bacteria, such as Strept. pyogenes 

 and M. pyogenes var. aureus produce haemolytic toxins. There are 

 certain protozoa, as, for example, the Amoeba dysenteries and the various 

 trypanosomes which may possibly secrete soluble poisons. Among the 

 animals, the venoms of the poisonous snakes, the poison of the centipedes, 

 and spiders, the serum of the eel and the excretion of the dermal glands 

 of the toad are examples of secreted toxins (zootoxins). Again, among 

 the plants are abrin from the jequerity bean, ricin from the castor-oil 

 bean, examples of soluble toxins (phytotoxins) the product of plant cells. 



