ANIMAL ORGANISMS IN HEALTH AND DISEASE 489 



term, since it would include the results of any overactivity or strain 

 leading to abnormal conditions in the body. 



4. Infective Organisms. — Infective organisms comprise both plant 

 and animal parasites which cause disease either by depriving the organ- 

 ism of something which it needs or by the creation of wrong living condi- 

 tions, including the development of poisons, in the body of the host, 



546. Effect of Individuality. — The individual character of the organ- 

 ism has a decided effect upon the susceptibility to disease. Naturally 

 this individuality may be a matter of inheritance but it also may be an 

 acquired characteristic. Particularly is this seen in the way individuals 

 react to articles of food. Some persons are unable to eat certain foods, 

 as, for example, acid fruits and the yolk of eggs, although to most people 

 the same foods are entirely innocuous. This individuality also affects 

 the responses which the body gives to certain drugs, and the degree of 

 toleration shown by different individuals toward drugs must be taken 

 into consideration in the treatment of disease. Any such peculiar 

 and individual responses to food or drugs are often known by physicians 

 as idiosyncrasy. It may be a form of allergy (Sec. 551). 



547. Self -regulatory Tendency in the Body. — A one-celled organism 

 subjected to modifications of its environment changes in such a way 

 as to adjust itself to the altered environment or, if it cannot meet the 

 changes, protects itself against them by encystment. In the same way 

 the cells of a many-celled organism tend to adjust themselves to changed 

 conditions within the body or, if they cannot so adjust themselves, resort 

 to certain means of protection from those conditions. These means 

 sometimes involve such extreme measures as the dropping off of a portion 

 of the body, or autotomy. A self-regulatory tendency is very fortunate 

 for man as well as for other organisms, since through it an animal will 

 of itself tend to regain its health when subjected to conditions that cause 

 disease, even though no assistance is given from without. 



548. Toxins and Antitoxins. — The living of one organism in the 

 body of another may impose certain hardships upon the latter. In 

 addition to its own wastes the body of the host must eliminate the wastes 

 produced by the other organism. Of course, if the relationship is one 

 of symbiosis the elimination of these wastes does not impose a hardship 

 sufficient to counterbalance the advantage accruing from the relation- 

 ship, but in the case of a parasite this may be a serious strain upon the 

 host. Whenever these wastes act as a poison in the body of the host 

 they are included under the general term of toxins. This term, however, 

 also includes all other poisons which may be introduced into the body, 

 whatever their source. 



To any such poison, either elaborated within the body by some 

 parasitic organism or introduced into the body in any manner, the body 

 reacts by producing a substance which tends to neutralize the poison 



