share their fate. If all of us are threatened alike by the invisible enemies in the air we breathe, how is it that so many escape? If we expose 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 inevitable 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 assaults, 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 contagious 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 matter 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 parasite 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,