Four criteria designed by Robert Koch to establish a causative relationship between a microbe and a disease.
Since initial formulation in 1884 these have been modified in the light of asymptomatic or healthy carriers of some diseases (e.g. Typhoid fever), and the difficulty of producing infection with some organisms (e.g Mycobacterium leprae), but remain a reasonable approximation to the requirements to satisfy before deciding an organism is the cause of a disease. His foresight and clear thinking did much to ensure the acceptance of the germ theory of disease.
- Identify an organism in cases of the disease
- Isolate the organism from those cases in pure culture
- Produce the same disease by administering the organisms to healthy individuals
- Isolate the organism from those individuals in who the disease has been induced.
They are still useful for example in identifying the cause of SARS 
Other criteria for assessing whether an association is likely to be causal have been developed.
Harvard University Historical website A brief history of an exciting period in the science of infectious disease
A photographic reproduction of a series of Lectures by Koch in 1909
- ↑ Osterhaus AD, Fouchier RA, Kuiken T. The aetiology of SARS: Koch's postulates fulfilled. Philosophical transactions of the Royal Society of London. Series B, Biological sciences. 2004 Jul 29; 359(1447):1081-2.(Link to article – subscription may be required.)
- ↑ http://ocp.hul.harvard.edu/contagion/koch.html accessed 28th March 2009
- ↑ http://pds.lib.harvard.edu/pds/view/6484229 accessed 28th March 2009