304 BIOLOGY AND HUMAN LIFE 



The activities of a parasite may cause injury to some particu- 

 lar organ and interfere with its functions so much as to affect 

 the health of the whole body. Pneumonia, rheumatism of the 

 heart, and cerebrospinal meningitis are examples of diseases in 

 which the parasite causes local injury. When the injury to one 

 of these vital organs goes too far, death results. 



234. Other classes of disease. Certain bacteria cause decay 

 of food material; with the production of poisonous wastes ; any- 

 body taking food that has undergone such changes will be tak- 

 ing the poison into his system directly. ]\Iost important for 

 human beings are the inorganic poisons lead, mercury, and 

 phosphorus, which are used in certain industries. In recent 

 years the use of white phosphorus in the manufacture of 

 matches has been completely discontinued because of the seri- 

 ous injuries which the fumes produced upon workers. 



A deficiency of essential elements or an excess of materials 

 may bring about abnormal behavior on the part of various or- 

 gans. Nutrition may also affect the activities of the ductless 

 glands, and so bring about derangement in the workings of the 

 essential organs. Among the nutritional diseases are rickets 

 (see Fig. 142), beriberi, pellagra, and scurvy. Deficiencies of 

 the thyroid, showing themselves in goiter and disturbances of 

 the nervous system, have been traced to a lack of iodin in the 

 food or water. 



235. Symptoms and causes. For many centuries people knew 

 nothing about disease except the suffering of the patient and the 

 outward manifestations. A sickness meant pains in various 

 parts of the body, and changes in the color of the skin, out- 

 breaks on the skin, fever or chills, sweating or dryness, perhaps 

 a peculiar odor, looseness of the bowels or constipation, and so 

 on. The object of treating or drugging was to get rid of the 

 symptoms : if a person was hot, he had to be cooled ; if he was 

 cold, he had to be warmed ; if he was dry, he must have water. 

 During the latter part of the nineteenth century the art and 

 the science of medicine were completely revolutionized through 

 the discoveries made by the French investigator Louis Pasteur 



