Koch's postulates (according to Robert Koch) are classic criteria for determining whether a microorganism causes a disease - they originate from Robert Koch (19th century) and apply in infection biology.
Koch's four original postulates are as follows:
1. the pathogen must be detectable in every case of disease, but not in healthy individuals.
2. it must be possible to grow the pathogen in pure culture
3. the pure culture must cause the same disease in animal experiments
4. it must be possible to isolate the pathogen from the infected animal.