Body & HealthMental model
Four rules that separate cause from coincidence.
Robert Koch and Friedrich Loeffler set out four conditions in 1884: the microbe is found in all the sick, can be grown in pure culture, produces the same disease in a healthy host, and can be re-isolated from it. Koch himself dropped the first when he found healthy cholera carriers. Viruses won't grow without host cells at all.
Useful: A checklist is powerful because it can fail – and the failures teach you something.
Source: en.wikipedia.org