A coma (or comatose state) was a state of unconsciousness in which a person was unresponsive to physical stimulation due to a reduction of neurological activity in the neocortex.
Time Lords could voluntarily go into a coma. (TV: Spearhead from Space; AUDIO: Time Tunnel) After being attacked by a giant rodent in Paxis Majoritis, the capital city of Synchronis, the Fourth Doctor placed himself in a coma in order to heal his injuries. (AUDIO: Empathy Games) The Seventh Doctor placed himself in a coma to prevent himself from being taken over by a Scourge. (AUDIO: The Shadow of the Scourge)
In 1871, the Seventh Doctor woke from a two-week coma to find himself in Paris. He was reunited with Bernice Summerfield and Ace. (PROSE: Set Piece) He later spent millions of years in a coma, suspended within a block of ice in Antarctica, from which he woke in 2012. (AUDIO: Frozen Time)
After spending most of the previous 24 years in a coma with only occasional moments of lucidity, former UNIT officer Francis Cleary died in his mother's arms on 5 April 1995 at the age of 46. (PROSE: Who Killed Kennedy)
When the Sixth Doctor found himself poisoned by a large dose of aspirin, which caused severe allergic, pulmonary and cerebral embolism in Time Lords, he fell into a deep coma while his body fought off the effects. (PROSE: Burning Heart)
On 24 December 2006, Daniel Francis Thompson went into a coma after being struck by a car and taken to the A&E department of St. Nicholas's Hospital. As Daniel lay in the coma, he somehow drew the Doctor's TARDIS into his subconscious mind in the form of his bedroom. A mental projection of Daniel, which only remembered wanting to see Christmas, entered the TARDIS and journeyed with the Tenth Doctor to several Christmases in history, including a World War I battlefield in Belgium, a party with guests from all over time and space and even the very first Christmas in Bethlehem. None of these satisfied Daniel. After returning to the TARDIS, he began to fade away. The Doctor realised what was happening, tracked down Daniel's physical body's location and helped the doctors at the hospital save the boy's life. (PROSE: Deep and Dreamless Sleep)