Chance

-ran out of hot oil on the battlefields by chance

-used his initative to make a new treatment

-unsure about whether it would work

-luckily it did

-by chance he came across the ancient folk law saying that cow pox prevented people from catching small pox

-Charles Chamberland accidentally left chicken cholera culture on his research desk and went on holiday

-air got into contact with the culture and attenuated (weakened) it

-they did not realise this when they injected the chicked with it

-the chickens with this culture survived

-so attenuated cholera culture had given the chicked immunity