Radioactivity is the loss of energy from an unstable nucleus of an atom. Radioactivity is produced by fissionable nuclear material and is harmful to living creatures that are exposed to it. But let's face it... this is television! Half the time the people that are exposed to excessive amounts of radiation either pick up some random hideous mutation or suddenly spawn super-powers.
Following a nuclear war between NATO forces and the Soviet Union, residents of Lawrence Kansas and Kansas City, Missouri are exposed to high levels of atomic radiation, which causes an epidemic of sickness and death.
High school nerd Peter Parker is bitten by a radioactive spider that gives him the proportate strength, speed and agility of a spider. It also grants him a precognitive warning of impending harm, which he refers to as his spider-sense.
The castaways came upon a crate of radioactive food that washed up on the shore of the deserted island they were stuck on. Not knowing any better, they planted crops with it and some of them gained temporary super-powers. Gilligan developed superhuman strength. Mrs. Howell developed superhuman stamina and Mary Ann gained enhanced vision.