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Everything is found! Now I have found out how to make a beast a little sick—just a little sick so that he will get better, from a disease. . . . All we have to do is to let our virulent microbes grow old in their bottles . . . instead of planting them into new ones every day. . . . When the microbes age, they get tame . . . they give the chicken the disease . . . but only a little of it . . . and when she gets better she can stand all the vicious virulent microbes in the world. . . . This is our chance—this is my most remarkable discovery—this is a vaccine I've discovered, much more sure, more scientific than the one for smallpox where no one has seen the germ. . . . We'll apply this to anthrax too . . . to all virulent diseases. . . . We will save lives . . . !"

A lesser man than Pasteur might have done this same accidental experiment—for this was no test planned by the human brain—a lesser man might have done it and would have spent years trying to explain to himself the mystery of it, but Pasteur, stumbling on this chance protection of a couple of miserable chickens, saw at once a new way of guarding living things against virulent germs, of saving men from death. His brain jumped to a new way of tricking the hitherto inexorable God who ruled that men must be helpless before the sneaking attacks of his sub-visible enemies. . . .

Pasteur was fifty-eight years old now, he was past his prime, but with this chance discovery of the vaccine that saved chickens from cholera, he started the six most hectic years of his life, years of appalling arguments and unhoped-for triumphs and terrible disappointments—into these years, in short, he poured the energy and the events of the lives of a hundred ordinary men.

Hurriedly Pasteur and Roux and Chamberland set out to confirm the first chance observation they had made. They let virulent chicken cholera microbes grow old in their bottles