Louis Pasteur by Nicola Kingsley - National STEM Centre

Early vaccination
for diphtheria
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bodies, of this microbe grown outside the body, could bring
about the disease. Once injected, the chickens rapidly
became ill and died.
On the day the laboratory was due to close for the summer
holidays, one of the assistants forgot to inject a batch of
chickens with the culture of microbes that had been
prepared. It was left standing in a cupboard for several
weeks. When the holiday was over, the assistant, picking
up where he had left off, went ahead and injected the
culture. The chickens receiving it became ill-but instead
of dying, quickly recovered. The assistant fetched Pasteur,
explained what had happened, and was about to throw
away the rest of the culture when Pasteur stopped him.
"In the field of experimentation", Pasteur once said,
"Chance favours only the prepared mind." It may be that
the memory of Jenner's vaccination had prepared Pasteur's
mind for this chance occurrence. His assistant might have
been ready to dismiss it as a hiccup in the experiment, but
Pasteur was not. He told him to inject the hens again, this
time with a fresh culture capable of giving a deadly dose of
the disease. The birds remained perfectly healthy. Someone
was sent to the market to buy some more chickens.
Injected with the same culture, these hens rapidly
developed chicken cholera and died.