62 MOUNTED INSTRUCTION 



Some outbreaks of infectious diseases appear very suddenly, with 

 the most virulent and rapidly fatal cases appearing earliest in the out- 

 break. The outbreak then gradually loses virulence, the last cases be- 

 ing of a decidedly chronic nature, and some of them possibly recover- 

 ing. It is possible however, that, before the virulent form was noticed, 

 there may have been a series of very mild cases, the animals not being 

 appreciably sick ; but the germs in passing through susceptible bodies 

 greatly increased in virulence until they were able to produce a rapidly 

 fatal type of the disease. 



Body Entrance: Germs gain entrance through the respiratory or- 

 gans with the inspired air; through the digestive organs ; through cuts 

 or scratches in the skin and mucous membranes. 



Method of Injury: Germs cause injury and disease in at least two 

 different ways : First, by rapid multiplication and direct attack in in- 

 conceivable numbers upon the tissue. As an example of condition 

 caused by germs in this way, we have fistulous withers. Second, germs 

 which by chemical action may produce intensely poisonous substances 

 in the blood and body tissues. As an example of this we have tetanus. 



How Disposed of in Nature: Nature disposes of disease germs in a 

 variety of ways, principally by oxidation, by the devitalizing effect of 

 sunlight, and by scattering them over wide areas by means of wind 

 and water. 



Bacteria or germs usually require very favorable conditions for exist- 

 ence, and especially for retaining disease-producing power. Make con- 

 ditions unfavorable for them by vigorous health, by high resisting 

 power on the part of the animal, and by sunshine and ventilation. 

 Bear in mind that germs are actual substances, tiny particles of living 

 matter, and may be carried about in any way that very fine particles 

 of dust may be scattered. 



The beginning of an outbreak may come in two ways : First, by 

 recent introduction of germs ; or second, an outbreak of infectious 

 disease may be caused by increased virulence in case of germs already 

 present, but not previously capable of producing disease. This in- 

 crease of virulence may be due to surrounding conditions especially 

 favorable to germ life. 



Apparently it makes great difference in many diseases as to the num- 

 ber of germs taken into the body, — hence the necessity of sunshine to 

 reduce virulence and destroy germs and of ventilation to carry as many 

 of them as possible out into the open air. 



DISINFECTION AND DISINFECTANTS 



Causes of Infection: In any process of disinfection it is important 

 to know something concerning the nature of the infecting germ and the 

 sources from which it comes. Germs of diseases are spread in a great 

 variety of ways ; for instance with the body fluids of sick animals, by 

 soiled water or food, or by any contaminated matter. Air may be con- 

 taminated from the skin and lungs of diseased animals. Thesoil may 

 be contaminated by the burial of diseased animals or by the deposit 



