Terminal illnesses are diseases or illnesses that either have no known cure or that have progressed beyond the stage of being treatable. Barring a miraculous recovery, such a disease is certain to end a sickened individual's life, though treatment options may still allow the afflicted to live longer than they might have without treatment.
Examples in Video Games
Metal Gear Solid
In the Metal Gear Solid series, Solid Snake becomes an unwitting carrier of a disease called FOXDIE. This scientifically engineered disease is programmed to attack targets with specific DNA patterns and mimics the symptoms of cardiac arrest. It is otherwise harmless to people whose DNA are not programmed targets. Numerous characters in Metal Gear Solid and Metal Gear Solid 4 fall victim to the disease. Snake himself was also an intended target, but Naomi Hunter had modified the strain injected into his system to allow him to live longer, though how much longer is not known.
The premise of Eternal Sonata is based on the final dreams of Frederic Chopin as he lays dying in bed of tuberculosis. Within his dream, he encounters a girl named Polka who is in a similar state and suffering from a terminal illness of her own. However, in Chopin's dream world, those afflicted with such maladies are imbued with great magical power.
Terminal illness is a fairly common trope in the Visual Novel genre, particularly the "Nakige" ("Crying Game") sub-genre. Various games in this sub-genre feature a character, often a tragic heroine, suffering from a terminal illness. Examples include Kana: Little Sister and Clannad.