humans. In the process, he’d come up with a set of rules – or ‘postulates’ – to identify whether a particular germ is responsible for a disease. To start with, he thought that it should always be possible to find the germ inside someone who has the disease. Then, if a healthy host – like a laboratory animal – was exposed to this germ, it should develop the disease too. Finally, it should be possible to extract a sample of the germ from the new host once they fall ill; this germ should be the same as the one they were originally exposed to.[40]
