The principle was revived again, however, by Pasteur, and this time with a logical thought as to its value.  While working upon anthrax among animals, he learned that here, as in other diseases, recovery, when it occurred, conveyed immunity.  This led him to ask if it were not possible to devise a method of giving to animals a mild form of the disease and thus protect them from the more severe type.  The problem of giving a mild type of this extraordinarily severe disease was not an easy one.  It could not be done, of course, by inoculating the animals with a small number of the bacteria, for their power of multiplication would soon make them indefinitely numerous.  It was necessary in some way to diminish their violence.  Pasteur succeeded in doing this by causing them to grow in culture fluids for a time at a high temperature.  This treatment diminished their violence so much that they could be inoculated into cattle, where they produced only the mildest type of indisposition, from which the animals speedily recovered.  But even this mild type of the disease was triumphantly demonstrated to protect the animals from the most severe form of anthrax.  The discovery was naturally hailed as a most remarkable one, and one which promised great things in the future.  If it was thus possible, by direct laboratory methods, to find a means of inoculating against a serious disease like anthrax, why could not the same principle be applied to human diseases?  The enthusiasts began at once to look forward to a time when all diseases should be thus conquered.

But the principle has not borne the fruit at first expected.  There is little doubt that it might be applied to quite a number of human diseases if a serious attempt should be made.  But several objections arise against its wide application.  In the first place, the inoculation thus necessary is really a serious matter.  Even vaccination, as is well known, sometimes, through faulty methods, results fatally, and it is a very serious thing to experiment upon human beings with anything so powerful for ill as pathogenic bacteria.  The seriousness of the disease smallpox, its extraordinary contagiousness, and the comparatively mild results of vaccination, have made us willing to undergo vaccination at times of epidemics to avoid the somewhat great probability of taking the disease.  But mankind is unwilling to undergo such an operation, even though mild, for the purpose of avoiding other less severe diseases, or diseases which are less likely to be taken.  We are unwilling to be inoculated against mild diseases, or against the more severe ones which are uncommon.  For instance, a method has been devised for rendering animals immune against lockjaw, which would probably apply equally well to man.  But mankind in general will never adopt it, since the danger from lockjaw is so small.  Inoculation must then be reserved for diseases which are so severe and so common, or which occur in periodical epidemics of so great severity,