“Now, Doctor,” said Pasteur, “choose two of these sick cows--we’ll call them A and B. Give them your new cure, and we’ll leave cows C and D without any treatment at all.” So Louvrier assaulted poor A and B with his villainous treatment. The result was a terrible blow to the sincere would-be curer of cows, for one of the cows that Louvrier treated got better--but the other perished; and one of the creatures that had got no treatment at all, died--but the other got better. “Even this experiment might have tricked us, Doctor,” said Pasteur. “If you had given your treatment to cows A and D instead of A and B--we all would have thought you had really found a sovereign remedy for anthrax.” Here were two cows left over from the experiment, beasts that had had a hard siege of anthrax and got better from it: “What shall I do with these two cows?” pondered Pasteur. “Well, I might try shooting a still more savage strain of anthrax bacilli into them--I have one family of anthrax germs in Paris that would give even a rhinoceros a bad night.” So Pasteur sent to Paris for his vicious cultivation, and injected five drops into the shoulders of those two cows that had got better. Then he waited, but nothing happened to the beasts, not even a tiny swelling at the point where he had injected millions of poisonous bacilli; the cows remained perfectly happy! Then Pasteur jumped to one of his quick conclusions: “Once a cow has anthrax, but gets better from it, all the anthrax microbes in the world cannot give her another attack--she is _immune_.” This thought began playing and flitting about in his head and made him wool-gather so that he did not hear questions that Madame Pasteur asked him, nor see obvious things at which his eyes looked directly. “How to give an animal a _little_ attack of anthrax, a safe little attack that won’t kill him, but will surely protect him.... There must be a way to do that.... I must find a way.” So it went with Pasteur for months and he kept saying to Roux and Chamberland: “What mystery is there, like the mystery of the non-recurrence of virulent maladies?” He went about muttering to himself: “We must immunize--we must immunize against microbes....” Meanwhile Pasteur and his faithful crew were training their microscopes on stuff from men and animals dead of a dozen different diseases; there was a kind of mixed-up fumbling in this work between 1878 and 1880--when one day fate, or God, put a marvelous way to immunize right under Pasteur’s lucky nose. (It is hard for me to give you this story exactly straight because all of the various people who have written about Pasteur tell it differently and Pasteur himself in his scientific paper says nothing whatever about this remarkable discovery having been a happy accident.) But here it is, as well as I can do, with certain gaps that I have had to fill in myself.