The book describes how Louis Pasteur was born into a Europe still plagued by horrendous diseases, such as cholera and typhoid, and how he discovered that it was germs living in the air and in food that caused illnesses. The reader learns that his pioneering of a method of heating food just enough to kill these germs gave the world 'pasteurisation'. Then, when Pasteur noticed that animals which had had anthrax never caught it again, and he injected other animals with a weak form of the disease and found that their bodies developed immunity to it, the reader learns how vaccination was invented and is now used to prevent many diseases, both in people and animals.