The cheetah virus (PROSE: Stop the Pigeon, Prime Time) was the "virus" of the living Cheetah World that created Cheetah People by contaminating other species and slowly transforming them into Cheetahs the more they fought. (TV: Survival)
It was a form of DNA corruption, related to the high amount of artron energy in the planet's magnetosphere that enabled the world to metamorphose living matter, (PROSE: First Frontier) described as everything on the planet, the rocks, trees, etc., having "an extra dimension." (PROSE: Survival)
The effects of the virus could be held back but became increasingly difficult, resurfacing under stress. Victims gained increased agility, physically transforming them with cat eyes, yellow fur, claws, (PROSE: Stop the Pigeon) and sharpened teeth. (PROSE: Prime Time)
A great civilisation colonised the Cheetah Planet, attempting to control the planet's nature, but nature infected them and the virus transformed them into the Cheetahs, making them part of the planet.
When the Master landed on the planet he caught the virus and gained control of the Kitlings, using them to teleport humans from Earth so he could use one of them to get off the planet, the virus transferring to a number of humans as well. When the Cheetah World was destroyed, the Seventh Doctor told Ace the survivors could still feel the planet inside them. While even Time Lord minds weren't strong enough to completely resist the virus, (TV: Survival) the Seventh Doctor theorised that the Master's corrupted Trakenite DNA was most affected. (PROSE: First Frontier) According to the Eleven, the Master used the Cheetah race as a way to unnaturaly prolonge his life. (AUDIO: Planet of Dust)