CHAPTER XXXV 
THE BIOLOGY OF DISEASE 1 
STUDENT REPORT 
How many in the class have been sick during the past year? Of 
how many different diseases? What was done to aid each one in 
getting well? What was done to prevent others from taking the 
same diseases? What was done by your Health officer? (Consult 
the reports of the State Board of Health and of the local health official.) 
371. Disease. — Usually people go through their daily 
occupations without feeling pain or bodily discomfort. 
Such a condition is known as health. Sometimes, how¬ 
ever, they go about their usual duties when they do not 
feel well and the indisposition gradually passes away. But 
in other cases the ill feeling becomes severe, the usual ac¬ 
tivities are given up, and we say that they are sick. Sick¬ 
ness may last for only a short time or for many years. The 
usual conditions of the body are changed, and we say that 
the body is diseased. The apple, the tree, the dog, the 
horse, each has its own diseases. 
372. Cause of Disease. — While there are many causes 
of disease, all of them may be grouped under four headings: 
(1) Inherited diseases, i.e. those transmitted from parent 
to child, as certain forms of insanity and imbecility. (2) Dis¬ 
eases caused by such poisons as lead, arsenic, mercury, phos¬ 
phorus, opium, cocaine, alcohol, and the like. The dis¬ 
turbances which these chemical agents set up in animal 
tissues are easily recognized by a good physician. (3) Dis¬ 
eases which cause certain tissues to take on an abnormal 
1 Chapter XXIII, Bacteria, may be read in connection with this chapter. 
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