80 CAUSES AND PREVENTION OF DISEASE. 



unfavorable conditions retain virulence. For example, the spores 

 of anthrax. 



Some outbreaks of infectious diseases appear very suddenly, 

 with the most virulent and rapidly fatal cases appearing earliest 

 in the outbreak. The outbreak then gradually loses virulence, 

 the last cases being of a decidedly chronic nature, and some of 

 them possibly recovering. This is frequently illustrated in hog 

 cholera. It is possible, however, that before the virulent form 

 was noticed, that 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. 



• COCGACEAE- 



9mm BACrtRIACE-AE- 

 ^^ (SPIRILLACtAE- TYP&5 



;- 



FIG. 32. GENERAL GROUPS OF BACTERIA. DIAGRAMMATIC. (If. //. A'.) 



Body entrance. — Germs gain entrance through the respiratory 

 organs with the inspired air ; through the digestive organs ; 

 through cuts or scratches in the skin and mucous membranes, and 

 rarely infection occurs l^efore birth. 



Method of injury. — Germs cause injury and disease in at 

 least two different ways, first, by rapid multiplication, and me- 

 chanical presence in inconceivable numbers. As an example of 

 disease caused by germs in this way, at least partly by mechanical 

 presence, we have actinomycosis (lumpy jaw). Second, germs 

 which by chemical action may produce intensely poisonous sub- 

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

 have tetanus and diphtheria. 



How destroyed in nature. — Nature disposes of disease 

 germs in a variety of ways, principally by oxidation, l)y the do- 

 vitalizing effect of sunlight, and they are scattered over wide areas 

 and enormously diluted by the wind and water. 



Classification. — Germs are classified into general families ac- 

 cording t(( fi)rm, niult'plicatiiMi, presence or absence of sheath 



