If the successive cultures of fowl cholera, made at short intervals, have such virulence that ten or twenty inoculated birds perish in the space of twenty-four or forty-eight hours, a culture which has remained, say, for three months in its flask, the mouth of which has been protected from the introduction of all foreign germs by a stopper of cotton wool, which allows nothing but pure air to pass through it--this culture, if used to inoculate twenty fowls, though it may render them more or less ill, does not cause death in any of them. After some days of fever they recover both their appetite and spirits. But if this phenomenon is extraordinary, here is one which is surely in a different sense singular. If after the cure of these twenty birds they are reinoculated with a very virulent virus--that, for instance, which was just now mentioned as capable of killing its hundred per cent. of those inoculated with it, in twenty-four or forty-eight hours--these fowls would perhaps become rather ill, but they would not die. The conclusion is simple; the disease can protect _from_ itself. It has evidently that characteristic of all virulent diseases, that it cannot attack a second time. However curious it may be, this characteristic is not a thing unknown in pathology. Formerly it was the custom to inoculate with small-pox to preserve from small-pox. Sheep are still inoculated to preserve them from the _rot_; to protect horned cattle from peripneumonia they are inoculated with the virus of the disease. Fowl cholera offers the same immunity; it is an additional scientific acquisition, but not a novelty in principle. The great novelty which is the outcome of the preceding facts, and which gives them a distinct place in our knowledge of virulent diseases, is that we have here to do with a disease of which the virulent agent is a microscopic parasite, a living organism cultivated outside of the animal body, and that the attenuation of the virulence is in the power of the experimenter. He creates it, he diminishes it, he does what he wishes with it; and all these variable virulences he obtains from the maximum virulence by manipulation in the laboratory. Looked at in juxtaposition with the great fact of vaccination for small-pox, this weakened microbe, which does not cause death, behaves like a real vaccine relatively to the microbe which kills, producing a malady which may be called benign, since it does not cause death, but is a protection from the same malady in its more deadly form.