488 
THE BIOLOGY OF DISEASE 
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. 
373. 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 not 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 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 and plant 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 
result 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. 
374. Communicable Diseases. — The term communicable 
disease 1 is used in this book to mean the diseases caused by 
a plant or animal living as a parasite in plants, animals, or 
1 New York State designates the following as communicable diseases : 
anthrax; chickenpox; cholera, Asiatic; diphtheria (membranous croup)- 
ysentery, amoebic and bacillary; epidemic cerebro-spinal meningitis; 
epidemic or streptococcus septic sore throat; German measles ; glanders • 
measles ; mumps ; ophthalmia neonatorum ; para-typhoid fever ; plague • 
poliomyelitis acute anterior (infantile paralysis); puerperal septicaemia;’ 
rabies , scarlet fever ; smallpox ; trachoma ; tuberculosis ; typhoid fever • 
typhus fever; whooping cough. 
