262 BACTERIA, YEASTS, AND MOLDS 



One thing to remember in the spread of disease, whether 

 from bacillus carriers or from active cases, is the ease with 

 which our usual habits allow us to contaminate our hands 

 with saliva or nasal discharges and then to transfer this 

 contamination to others. Many sanitarians insist that this 

 is the most important form of direct contagion, and that 

 if we could all manage to keep our hands away from mouth 

 and nose, the spread of disease would be reduced to a 

 minimum. Unfortunately, hoT^ever, in the case of a head 

 cold, when the danger of infection is greatest, it is almost 

 impossible to avoid thus contaminating the hands. We 

 should, nevertheless, learn to regard our hands as con- 

 taminated at such times and should be careful not to 

 handle food or to touch other people (as in shaking hands) 

 without first washing our hands. 



3. Means of Invasion 



A matter of almost equal importance in considering the 

 distribution of disease is the means by which the bacteria 

 get into the body. Each species may have its own means 

 of entering, and frequently each can find entrance in only 

 one way. If it should get in by other means it would pro- 

 duce no injury. Some species, however (tuberculosis), pro- 

 duce trouble, no matter how or where they enter. K we 

 know the means of entrance of any contagious material, 

 we are of course in a much better position to guard our- 

 selves against it. The important means of entrance are 

 as follows. 



The Skin. Some diseases find entrance chiefly through 

 the skin. This is true of the bacteria which cause the 



