The cheetah virus (PROSE: Stop the Pigeon) was the "virus" of the Cheetah World that created Cheetah People, described as everything on the planet, the rocks and trees, having "an extra dimension." (PROSE: Survival) It contaminated other species and slowly turned them into new Cheetahs the more they fought. 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) It was difficult to hold the effects of the virus back, and it would resurface under stress, giving the victim cat eyes and yellow fur and claws. (PROSE: Stop the Pigeon)
A great civilisation colonised the Cheetah World, but many succumbed to the virus and the survivors were wiped out by the Cheetah People. Once individuals became Cheetahs, they were 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)