COMMUNICABLE DISEASES. 203 



interval of space. Itch, for example, is contagious, but not in the least 

 infectious, whereas whooping-cough is infectious, but not contagious. 

 Some diseases are both contagious and infectious, as small-pox and diph- 

 theria. Malaria and yellow fever are infectious, but not in the least con- 

 tagious. However, the distinctions between infectious and contagious are 

 often not very clear. It would be better to discontinue these terms and 

 say that certain diseases are communicable from man to man or from animals 

 to man. When a disease picks its victims rather promiscuously, in a cir- 

 cumscribed area, with none of the usual characteristics of a contagion or 

 infection, we usually apply the term epidemic. For example, cerebro- 

 spinal meningitis and pneumonia may be epidemical. Diphtheria is often 



FIG. 77. Bacillus anthracis. This bacillus is spore-forming and causes the cattle 

 disease known as anthrax. This disease is especially common among sheep and cattle 

 and may be transmitted to man, especially those working with the wool, hides and 

 meat of infected animals. The two chief forms of anthrax in man are malignant pustule 

 and woolsorter's disease. The dried spores of this bacillus will live for years and will 

 withstand the boiling temperature for hours. Vaccinating animals against anthrax is 

 commonly practised now. Anthrax is frequently confused with glanders, an equine 

 disease caused by the Bacillus mallei, a, Non-spore-bearing bacilli; b, chains of cells; c, 

 spore-bearing bacilli. Cell-walls and plasmic contents are stained, the spores are unstained. 



epidemic in a community^ and as above stated, it is likewise infectious and 

 contagious. The term epidemic is, however, also applied to any communi- 

 cable disease which has become general in a given community. A more or 

 less common or spreading disease which is limited to and recurs in a given 

 district or country is said to be endemic in that district or country. En- 

 demics are usually due to climatic conditions which encourage certain 

 microbic and other disease-producing invasions. 



The causes of disease are of two kinds, primary or inciting and secondary 

 or predisposing. The primary cause of a disease is that factor or influence 

 which must invariably be active before the disease can possibly develop. 

 For example, the primary cause of diphtheria is the diphtheria bacillus; the 

 predisposing causes are exposure to wet and cold, impoverished condition 



