LIGHT AND DISEASE 133 



Not all microorganisms are harmful ; some are our friends 

 and are as helpful to us as are cultivated plants and domesti- 

 cated animals. Among the most important of the microor- 

 ganisms are bacteria, which include among their number both 

 friend and' foe. In the household, bacteria are a fruitful 

 source of trouble, but some of them are distinctly friends. 

 The delicate flavor of butter and the sharp but pleasing taste 

 of cheese are produced by bacteria. On the other hand, 

 bacteria are the cause of many of the most dangerous diseases, 

 such as typhoid fever, tuberculosis, influenza, and la grippe. 



By careful observation and experimentation it has been 

 shown conclusively that sunlight rapidly kills bacteria, and 

 that it is only in dampness and darkness that bacteria thrive 

 and multiply. Although sunlight is essential to the growth 

 of most plants and animals, it retards and prevents the growth 

 of bacteria. Dirt and dust exposed to the sunlight lose their 

 living bacteria, while in damp cellars and dark corners the 

 bacteria thrive, increasing steadily in number. For this rea- 

 son our houses should be kept light and airy ; blinds should 

 be raised, even if carpets do fade ; it is better that carpets and 

 furniture should fade than that disease-producing bacteria 

 should find a permanent abode within our dwellings. Kitch- 

 ens and pantries in particular should be thoroughly lighted. 

 Bedclothes, rugs, and clothing should be exposed to the sun- 

 light as frequently as possible ; there is no better safeguard 

 against bacterial disease than light. In a sick room sunlight 

 is especially valuable, because it not only kills bacteria, but 

 keeps the air dry, and new bacteria cannot get a start in a 

 dry atmosphere. 



