98 VETERINARY STUDIES 



sisting or spore stage, may live for very long periods of time, 

 even years, and under very unfavorable conditions retain viru- 

 lence, as do the spores of anthrax and black-leg. 



Some outbreaks of infectious diseases appear very suddenly, 

 the most virulent and rapidly fatal cases appearing at the begin- 

 ning. The outbreak then gradually loses virulence, the last 

 cases being of a decidedly chronic nature. This is frequently il- 

 lustrated in hog cholera. 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 with the inspired air 

 through the respiratory organs; through the digestive organs; 

 and through cuts or scratches in the skin and mucous mem- 

 branes; and, rarely, infection occurs before birth. 



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

 least two different ways : First, by rapid multiplication and 

 mechanical presence in inconceivable numbers, obstructing the 

 capillaries. As an example of disease caused by germs in this 

 way, at least partly by mechanical presence, we have anthrax 

 and, second, more common and serious, germs may produce in- 

 tensely poisonous substances called toxins, in the blood and 

 body tissue. As an example of this we have tetanus and 

 diphtheria. 



One class of bacterial toxins are apparently liberated in the 

 body fluid, as produced in tetanus (lockjaw). Another class 

 of bacterial poisons are held in the bodies of bacteria producing 

 them until the bacterial cell is destroyed, as in glanders. These 

 poisons injure or kill in various ways just as do other chemical 

 poisons, tetanus toxin, for instance, which causes excessive stim- 

 ulation of muscle fibers. Others cause fatal depression of vital 

 organs as in human diphtheria. 



Often several species of bacteria cooperate in producing the 

 original disease, as in hog cholera or the pus pockets of actino- 

 mycosis. There may occur a secondary invasion by a different 

 species. The second invader may add to the seriousness and 

 even change the features of the disease although it may be in- 

 capable of causing the original disease alone. This condition is 

 common in tuberculosis and hog cholera. 



