During the 22nd century, the Valakians suffered from a plague that was killing off its entire population, causing twelve million deaths in 2149 alone. As of 2150, they had already made first contact as Valakis had been visited by two warp-capable species, the Ferengi and the M'klexa. In 2150, the Valakians dispatched four sub-warp spacecraft to seek help from other, more advanced, warp-capable species.
The Enterprise encountered one of those ships in 2151 and rendered medical aid to the incapacitated crew. When the crew explained that their pre-warp race had nonetheless already made contact with ships from passing alien races, the Enterprise command crew decided that it was acceptable to openly visit Valakis (the danger of cultural contamination had already occurred, regardless of what Enterprise did).
Enterprise offered to aid in search for a cure and while they were visiting the planet, the ship's doctor, Phlox, discovered that the epidemic wasn't being caused by a virus or bacteria. Instead, the illness was genetic: the proteins that bound to Valakian chromosomes were deteriorating. This had been going on for thousands of years, but the rate of mutation had accelerated over the past few generations. Based on his projections, Phlox predicted that the Valakians would be extinct by the 24th century.
Furthermore, Phlox discovered that the Menk had the potential to become the dominant species on the planet. The Enterprise crew realized they could not interfere with the natural evolution of the two species on Valakis but agreed to offer drugs that would ease the symptoms of the disease. Because he thought the Valakians could not deal with warp technology on their own yet, Enterprise Captain Jonathan Archer declined to give warp technology to the Valakians for the purpose of finding a cure. (ENT: "Dear Doctor")