Radiation sickness was an illness caused by exposure to excess radiation. Coughing was one of the first signs of the illness. (AUDIO: Protect and Survive) According to Ram Singh, its symptoms were "agony, followed by death". Matteusz Andrzejewski agreed. (TV: Detained)
During the waning years of the Thousand Year War on Skaro, a young Davros discovered several birds who had died from radiation poisoning and fallen out of the sky. This led him to study the effects of radiation on DNA, research that ultimately led to the creation of the Daleks. (AUDIO: Innocence)
When the First Doctor, Susan Foreman and Ian Chesterton discovered, through the use of a Geiger counter, that there was excess radiation on Skaro, they deduced that it was causing radiation sickness. In that particular case, the effects of the disease included a jungle being turned to stone, soil becoming barren, and a feeling of sickness between the three. (TV: "The Survivors")
In Klimtenburg during the 19th century, fragments of the Hapthoid reactor unit of a crashed Cyber-ship were collected and made into jewellery. Because of the unknown nature of radiation, this led to an outbreak of Hapthoid Radiation Sickness before the Eleventh Doctor gathered up the fragments. The villagers believed it was a re-emergence of the bubonic plague caused by the Plague Warriors. (PROSE: Plague of the Cybermen)
In September 1939, Heinrich Himmler's castle of Drachensberg was destroyed after a primitive nuclear reactor in its basement went critical. People who neglected to leave the area began dying of radiation sickness, although nobody was aware of this and believed the ruins to be cursed. (PROSE: Timewyrm: Exodus)
On Oseidon, the Fourth Doctor and Sarah Jane Smith speculated that the soldier they saw going over a cliff was suffering from radiation sickness. (TV: The Android Invasion)
On Terminus, Bor suffered from radiation sickness. (TV: Terminus)
The Tenth Doctor intentionally gave himself severe radiation poisoning to save Wilfred Mott from exposure to it, triggering his regeneration into the Eleventh Doctor. (TV: The End of Time)
According to Owen Harper, symptoms of radiation sickness in humans included vomiting and intestinal bleeding. Prolonged exposure could cause a person's hair to fall out. (PROSE: Another Life)
When Grant told Spider-Man's origin story (gaining spider powers upon being bit by a radioactive spider), the Twelfth Doctor assumed that this had simply given the fictional hero radiation poisoning. (TV: The Return of Doctor Mysterio)