The Titan Disease is an extremely potent and deadly blood disease that originated from Dr. Penelope Young's research into the super-steroid, Venom and it's powerful successor, the Titan Formula. This disease would ultimately claim the life of The Joker.
Arkham Asylum Incident
Although Dr. Young realized the dangers, the Joker, who had been secretly funding the project as part of his own plans, became determined to unleash an army of monsters upon Gotham City to destroy it. Luckily for the citizens of Gotham, Batman was able to put a stop to the Joker's insane plans before they could get that far - but not before the Joker injected himself with a sample of the Titan Formula.
Arkham City Incident
A year later, while he was searching Arkham City for Joker, Batman was ambushed by Harley Quinn and knocked out. After he woke up, Batman discovered that the Joker was suffering from a horrible side effect of the Titan Formula, which gave him a fatal disease that was progressively killing him. At the time of Arkham City, the Joker only had a few hours left to live. Not willing to die without taking as many people with him as possible, the Joker had his poisoned blood shipped out of Arkham City and into every emergency room in Gotham.
In addition, the Joker infected Batman with his blood as well in an attempt to force the vigilante to find a cure for him. For the rest of the night, Batman went through seemingly impossible odds to try to find a cure to save himself and Gotham from certain death. Batman eventually found a way to create an antidote with the help of Mr. Freeze. The cure required a sample of Ra's al Ghul's blood. Just when Gotham's hope was about to be in Batman's hands, it was cruelly snatched from him by Harley Quinn who stole the cure from Freeze's safe and took it to Joker.Seemingly cured of his illness while the Dark Knight was rapidly beginning to succumb to his, the Joker put his plan to stage a violent break-out out of Arkham City to once and for all destroy Gotham City and everyone in it. Determined to put a stop to the Joker's plans before he caused anymore death and destruction, Batman made his way to the Joker's Funhouse at the Sionis Steel Mill where he finally confronted his arch-nemesis. However, before Batman could take the Joker down, they were interrupted by Hugo Strange's Protocol 10.
In the resulting explosion, Batman was pinned down by rubble, which left him vulnerable to the Joker, who planned to slit the Dark Knight's throat. Before the psychopath could make good on his threats, Joker was interrupted by Talia al Ghul, who offered to take him to the Lazarus Pit where he could become immortal and all-powerful in exchange for him sparing Batman's life. Despite Batman's protests that Talia didn't realize who she's dealing with, Joker brutally knocked Batman out and the two then departed to the Lazarus Pit. Later, after he was freed from the rubble by Catwoman, and stopped Strange and Protocol 10, Batman made his way to the Monarch Theatre where he witnessed Talia seemingly kill the Joker with her sword through his back and out through his stomach.
Unfortunately, Talia gravely underestimated her enemy's cunning, and was brutally shot in the back in cold blood from behind by the real Joker who had been using the shape-shifting villain, Clayface as a decoy.
Batman and Clayface then engaged in a brutal fight to get the Titan cure which Talia had stolen earlier from Harley Quinn and then dropped when she was shot. The Joker, still dying from his illness, used explosives to blow up the floor beneath Batman and Clayface's feet, which revealed that the Lazarus Pit had been hidden in the process. Still battling, Batman was able to eventually beat Clayface, retrieved the cure, and drank half of it, which cured himself. The Joker, meanwhile, was able to reach the Lazarus Pit and was about to dive in, when Batman used Talia's Sword to destroy the machinery, which was able to push Clayface into the Pit and destroyed it.
Batman eventually woke up from the resulting explosion, and was taunted by the Joker who had also survived. After the Joker taunted Batman about killing Talia and poisoning the Gotham hospitals with his blood, containing the Titan Disease, and Batman voiced his contemplation whether he should give Joker the cure or let him die from his disease, Joker suddenly attempted to take the cure from Batman by force, only for Batman to accidentally drop it to the ground in the process, rendering it destroyed. After Batman revealed that he was going to give the cure to the Joker even after all his horrible and murderous acts, the Joker started laughing at the irony before succumbing to his disease and dying as his arch nemesis watched.
- The Joker (Died after succumbing to the Titan Disease.)
- Batman (Became afflicted with the disease after Joker injected him with his own poisonous blood; cured himself from the fatal Titan disease effects after drinking from the Titan disease cure, although he did not manage to cure himself from Joker's blood, which mutated over time, and as a consequence, he became afflicted with the Joker Infection.)
- Poison Ivy (She cured herself due to her unique DNA)
Although Titan was intended to help patients, it quickly became obvious that the risks far outweighed the potential benefits as they would quickly transform into seemingly mindless beasts of incredible strength and speed.
- The cure that Mr. Freeze worked on during Arkham City only partially worked: it saved Batman's life, but didn't affect his blood. While Batman's blood wasn't lethal anymore, it still caused wild hallucinations and schizophrenic mood swings. Batman eventually overcame the effects by using a combination of Scarecrow's Fear Toxin and willpower. As they had no cure, the infected Jokers would have most likely died due to their poisonous blood, as their faces showed similar symptoms as the Joker did before he died.