A planet so free of disease that its people cannot die, leaving it crowded beyond endurance in The Mark of Gideon.
Gideon is a planet that was once a paradise. Its atmosphere is germ-free and its people regenerate, so they live long lives and seldom die. Their love of life and their beliefs reject contraception, and the planet has become so crowded that people are pressed together everywhere.
In The Mark of Gideon, the council abducts Kirk, intending to use his blood to introduce disease. Kirk is placed on a replica of the Enterprise, while Spock argues with Starfleet and the Gideonites for permission to beam down.