A viral regression was a type of medical analysis, mapping out the life cycle of a virus to calculate how long it took for it to achieve its present level of concentration, thus deducing the date of infection.
In 2375, Julian Bashir used this technique to find out when Odo was infected with the morphogenic virus. Shockingly, he discovered that he wasn't infected by the Female Changeling as he had previously assumed, but much earlier while undergoing tests at Starfleet Medical. This was a smoking gun proving that the virus was intentionally engineered by someone in the Federation, quickly guessed to be Section 31. (DS9: "When It Rains...")