COMMUNICABLE DISEASES 233 



easily recognized by a good physician. (3 ) I diseases which 

 cause certain tissues to take on an abnormal growth, as in 

 tumors and cancers. (4) Diseases caused directly or 

 indirectly by some definite living plant or animal. Such 

 diseases are called " biological diseases," because the source 

 or cause is in all instances some definite living plant or 

 animal. In our ordinary daily speech we often speak of 

 such ills as " germ " diseases. 



175. Biological Diseases. — The rattlesnake secretes a 

 poison which is forced through fangs or hollow teeth into 

 the blood of its prey. This poison affects the heart and 

 may result in death. One of the common and beautiful 

 mushrooms produces a similar poison which is nut de- 

 stroyed by cooking. If this particular mushroom is eaten, 

 death is almost certain to follow in from twenty-four to 

 forty-eight hours. In both of these cases the animal or 

 plant is large enough to be seen and easily recognized. 



But there are a considerable number of microscopic 

 plants and a few microscopic animals that have formed the 

 habit of living for at least a part of their life in other 

 plants and animals. During this time, as we have seen in 

 the study of animal parasites, they usually secure all, or the 

 greater part, of their food from the plant or animal in 

 which they are living. Two general causes of disease re- 

 sult from this parasitic habit. The parasite may destroy 

 certain cells of the body, or the material thrown off from 

 the body of the parasite may act as a specific poison. 



176. Communicable Diseases. — The term communicable 

 disease 1 is used in this book to mean the diseases caused by 



1 New York State designates t be following as communicable diseases: 

 anthrax; chickenpox; cholera, Asiatic: diphtheria (membranous croup); 

 dysentery, amoebic and bacillary ; epidemic cerebrospinal meningitis; epidemic 



or streptococcus (septic) sore throat: German measles: glanders; measles; 

 mumps; ophthalmia neonatorum; para-typhoid lever: plague; poliomyelitis, 



