A planet-wide genetic illness afflicting the Valakians in 2151, for which Doctor Phlox found a possible cure that he and Captain Archer decided to withhold.
The Valakians were a pre-warp people who sent ships into space seeking help against a disease that was killing millions. Enterprise NX-01 met one such ship and went to their world in the episode Dear Doctor.
Phlox found that the illness was not an infection but a flaw in the Valakian genome that would end the species within about two centuries. He also saw that the Menk, a second humanoid people living contentedly under Valakian care, were on the brink of an evolutionary advance that Valakian dominance was holding back.
Phlox argued against interfering. Archer, with no directive yet to guide him, agreed, and gave the Valakians only medicine to relieve their symptoms. The episode is often cited as a forerunner of the Prime Directive.