THE GERM-THEORY OF DISEASE. 583 



share their fate. If all of us are threatened alike by the invisible ene- 

 mies in the air we breathe, how is it that so many escape ? If we ex- 

 pose a hundred flasks of meat-broth to the same atmosphere, they will 

 all become tainted alike, and in the same time. But the animal body 

 is not a dead soil in which bacteria can vegetate without disturbance. 

 Though our blood and juices are the most perfect food the parasites 

 require, though the animal temperature gives them the best conditions 

 of life, they must still struggle for their existence with the cells of the 

 animal body. We do not know yet in what way our tissues defend 

 themselves, but that they do resist, and often successfully, is an inevi- 

 table conclusion. We can show this resistance experimentally in some 

 cases. The ordinary putrefaction-bacteria can thrive excellently in 

 dead blood, but if injected into the living blood-vessels they speedily 

 perish. Disease-producing germs, however, are better adapted to the 

 conditions they meet with in the body they invade, and hence they 

 can the longer battle with their host, even though they succumb in 

 the end. 



The resistance or want of resistance which the body opposes to its 

 invaders is medically referred to as the predisposition to the disease. 

 What the real conditions of this predisposition are, we do not know. 

 Experience has simply shown that different individuals have not an 

 equal power to cope with the parasites. Here, as throughout all 

 nature, the battle ends with the survival of the fittest. The invaders, 

 if they gain a foothold at all, soon secure an advantage by reason of 

 their terrific rate of increase. In some instances they carry on the 

 war by producing poisonous substances, in others they rob the animal 

 cells of food and oxygen. If the organism can withstand these as- 

 saults, can keep up its nutrition during the long siege, can ultimately 

 destroy its assailants, it wins the battle. Fortunately for us, victory 

 for once means victory forever, at least in many cases. Most conta- 

 gious diseases attack an individual but once in his lifetime. The 

 nature of this lucky immunity is unknown. The popular notion, that 

 the disease has taken an alleged "poison" out of the body, has just as 

 little substantial basis as the contrary assumption that the parasites 

 have left in the body a substance destructive to themselves. It is not 

 likely, indeed, that an explanation will ever be given on a purely 

 chemical basis, but in what way the cells have been altered so as to 

 baffle their assailants in a second attempt at invasion is as yet a mat- 

 ter of speculation. Unfortunately for us, there are other diseases of 

 probable bacterial origin, which do not protect against, but directly 

 invite, future attacks. 



A question now much agitated is, whether each kind of disease- 

 germs amounts to a distinct and separate species, or whether the para- 

 site of one disease can be so changed as to produce other affections as 

 well. When investigations on bacteria were first begun, it was taken 

 for granted that all bacterial forms, yeast-cells, and mold-fungus, 



