The four criteria for establishing a causal relationship between a microbe and a disease are called Koch's postulates.
Robert Koch and Friedrich Loeffler formulated the postulates in 1884, building on earlier ideas associated with Jakob Henle. Koch later refined and published them in 1890. The criteria required finding the suspected organism in diseased hosts, isolating it in pure culture, reproducing the disease in a healthy experimental host, and recovering the same organism again.
The framework helped establish the bacterial causes of diseases such as tuberculosis and cholera during the rise of germ theory. It gave scientists a systematic way to distinguish a true pathogen from an organism merely found near diseased tissue.
The postulates are not universal today. Viruses cannot always be grown in pure culture, and many infections have asymptomatic carriers. Koch himself modified the first postulate after studying cholera and typhoid. “Molecular Koch's postulates” later adapted the idea to test whether particular microbial genes contribute to disease.