(covers information from several alternate timelines)
A terminal illness or terminal condition was an incurable disease that ended with the death of the patient. Caring for those afflicted with these types of diseases could be difficult, particularly if one was the patient's sole source of comfort. (DS9: "Ties of Blood and Water")
In 2364, Jean-Luc Picard described the Aldean civilization as being terminally ill, as they were incapable of reproducing. (TNG: "When The Bough Breaks")
In 2368, Picard asked William T. Riker whether he would see death as a release if he was facing a terminal, incurable illness (TNG: "Ethics")
Later that year, Deanna Troi compared Data's finding his own severed head to discovering that a loved one had a terminal illness. (TNG: "Time's Arrow")
Acute anxiety could result upon one's diagnosis with a terminal illness. In 2376, The Doctor described Lewis Zimmerman as suffering from anxiety related to his terminal illness. (VOY: "Life Line")
In an alternate timeline in which Kathryn Janeway needed to help her past self return USS Voyager to the Alpha Quadrant sooner, Harry Kim claimed he had to take her to Starfleet Medical because she had a rare illness. Janeway quipped that she hoped it wasn't terminal. (VOY: "Endgame")
Terminal illnesses and conditions
- Cardiomyopathy (in the 1990s) (TNG: "The Neutral Zone")
- Contusion of the auricular lobe. (In Ferengi, allegedly, according to Julian Bashir (DS9: "Invasive Procedures"))
- Darnay's disease (TNG: "The Schizoid Man")
- Dislocation of the malleus (In Ferengi, allegedly, according to Julian Bashir (DS9: "Invasive Procedures"))
- Dorek Syndrome (DS9: "Body Parts")
- Dramia II plague (upon victims turning green) (TAS: "Albatross")
- Emphysema and liver damage (in the 1990s) (TNG: "The Neutral Zone")
- Infection of the tympanic membrane (In Ferengi, if left untreated) (DS9: "The Nagus", "Bar Association")
- Metremia (VOY: "Jetrel")
- Pulmonary embolism (in the 1990s) (TNG: "The Neutral Zone")
- Rigelian fever (in a single day if not treated) (TOS: "Requiem for Methuselah")
- Sakuro's Disease (if not treated) (TOS: "Metamorphosis")
- Ulcartic virus (DS9: "The Emperor's New Cloak")
- Veridium Six poisoning (TNG: "Reunion")
- Yarim Fel Syndrome (DS9: "Ties of Blood and Water")
- Xenopolycythemia (according to to Federation knowledge, prior to 2268) (TOS: "For the World is Hollow and I Have Touched the Sky")