patrick.net

Traditionally, proving that a microbe causes disease meant satisfying Koch’s postulates — four criteria laid out in the 19th century:

1. The microorganism must be found in all cases of the disease
2. It must be isolated and grown in pure culture
3. The cultured organism must cause the disease in a healthy host
4. It must then be re-isolated from the newly diseased host

Koch’s postulates were not fulfilled for SARS‑CoV‑2 in the classical sense.

Modern virology has shifted away from these criteria, favouring molecular techniques such as PCR, genome sequencing, and cytopathic observation in cell culture. But these newer tools do not prove causation — they indicate correlation and presence.