This would prevent epidemics. While studying fowl cholera, Pasteur noticed that a culture of the germ that was only a few months old did not make the chickens ill but instead protected them from the illness.
लेकिन सम्बन्धित जीवाणु प्रयोग करने के बजाय, पास्चर उसी रोग के तनूकृत कारक को प्रयोग करनेवाला पहला व्यक्‍ति था ।