ANIMAL ORGANISMS IN HEALTH AND DISEASE 517 



3. Traumatism. — In a popular sense traumatism is synonymous with 

 the term accident, but it is, properly speaking, a somewhat more inclusive 

 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 organism 

 of something which it needs or by the creation of wrong living conditions, 

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



Some infectious diseases, such as scarlet fever, yellow fever, infantile 

 paralysis, and influenza, are caused by viruses. Viruses may also cause 

 diseases in other animals or in plants. It has been known for some time 

 that a virus is composed of particles too small to be seen with the micro- 

 scope. Recently, Dr. Wilham Stanley of the Rockefeller Institute has 

 isolated the virus of tobacco mosaic disease and found it to be a protein 

 with a very large molecule. Although it can be crystallized and is 

 apparently nonliving under some conditions, when it is applied, much 

 diluted, to a living tobacco plant, it multiplies or increases rapidly as if 

 it were living. Viruses are thus most interesting biologically and 

 chemically since they seem to be intermediate between living and non- 

 living matter. 



548. 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. 553). 



549. 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 wdthin 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. 



