Before Arkham City
Prior to the events of Batman: Arkham City, the chances for a cure to the Titan Disease were looking slim. Not wanting to go out alone, the Joker had the doctors Strange sent him drain and store his infected blood, while transfusing healthy blood to extend his life.
Joker managed to blackmail Strange into sending his blood to hospitals all over Gotham City, by reminding him that he knew about his illegal experiments with Lunatics, the Mad Hatter and Quincy Sharp. Joker even managed to poison Batman with his blood, forcing the Dark Knight into helping him get a cure for the disease. Unfortunately for the Joker, when Batman had gained the cure and took his share, the Clown Prince of Crime caused him to drop the vial of cure; accepting his death, the Joker died with a smile on his face.
Before Arkham Knight
Although Batman managed to find and distribute a cure to the infected and rounded up most of Joker's blood samples with the help of Commissioner Gordon, four people were given blood transfusions with blood samples they missed due to hospital errors. It was sometime afterwards, that those individuals began showing signs of the Joker Infection, leading to their discovery and incarceration by Batman.
Batman linked the infection to Creutzfeldt-Jakob disease (or CJD) but it was mutated beyond anything on medical record. Like CJD, the Joker Infection attacked the brain and damaged the brain tissue, affecting the victim's mood and behavior. Unlike CJD however, this unique infection was not fatal, and instead it slowly made the host inherit the personality, traits and behavior of the Joker until they likely would eventually become a literal copy of the Clown Prince of Crime. (Although the disease could not change their body structure or gender.)
Arkham VRBatman, after repeated nightmares, feared that the infection would take him (as he had the mad clown's blood in him as well), keeping a quarantine cell ready. To his own horror, he found Scarecrow's new strain of toxin could exaggerate the Joker Infection's symptoms and its effects to the point of causing hallucinations. In Batman's case, a vivid hallucination of the Joker appeared that represented the embodiment of his worst fears.
So far, the only known way to cure one's self of the disease is to fight the infection within one's mind. The infection took the form of the Joker, who Batman was finally able to scare and lock away in a dark corner of his mind, purging himself of the Joker's insanity for good. In that case, it is heavily implied that Batman exploited the use of Scarecrow's Fear Toxin to ensure he could take on the infection. Other than that, a merciful death would be needed.
Aside from the obvious bit of adopting more Joker-like traits and to a certain extent hallucinating the Joker, the disease also results in the infected having intrusive flashbacks to the heinous actions that the Joker had caused in the past, regardless of whether the infected had ever actually witnessed such actions. In Batman's case, it was the crippling of Batgirl as well as bits and pieces of Joker's torture of Jason Todd, at that time the current Robin.
- Christina Bell (Killed by Henry Adams.)
- Johnny Charisma (Killed by Henry Adams.)
- Albert King (Killed by Henry Adams.)
- Henry Adams (Suppressed most (if not all) of the effects to fool Batman before being revealed, committed suicide.)
- Batman (Used sheer iron will and possibly Fear Toxin to combat the infection and finally remove Joker from his thoughts and lock him away forever, preventing himself from becoming the Joker.)
- It was speculated that the very first time that Batman showed symptoms of the Joker Infection was right after he defeated Mr. Freeze in Arkham City, as he hallucinated the Joker's face in place of Freeze's while subduing him. This was actually later confirmed by Sefton Hill in a "Ask me anything" event in reddit.
- Ironically, the infection enhanced a quality the victim already had.
- Christina Bell: suffered a miscarriage, thus losing a baby; she became fixated on Batman
- Johnny Charisma: a famous singer; the infection boosted his egomania
- Albert King: boxer; he became more violent
- Henry Adams: biology teacher; he believed in "survival of the fittest"
- Batman: More ruthless, but refrained from killing (although he implies [see below] that even his restraint on killing might disappear if he remained infected long enough).
- Most of the infected resemble alternate versions of the Joker from comic-books.
- Christina Bell: Is visually modelled after the female Joker, Bianca Steeplechase from Earth-37 (Dc's Thrillkiller).
- Johnny Charisma: Appears to be modelled after the Joker of Earth-31 (Batman: The Dark Knight Returns). He dresses in a white suit like Joker from this story and is a singer which could be an allusion from the Earth-31 Joker being modelled after David Bowie.
- Henry Addams: Henry looks like an elderly Joker which could be an allusion to the Joker of Earth-22 from DC's Kingdom Come.
- It was never fully revealed what the exact cause for the creation of the Joker Infection was other than it originating from Joker's mutated blood. However, it was probable that the chemicals that caused the Titan Disease (and maybe chemicals already in the clown's blood) caused Joker's blood to mutate, creating the Joker Infection.
- Aside from the two names stated above, Joker referred the illness as Joker-itis and Mad Clown Disease (a parody on Mad Cow Disease).
- It's shown that, despite having cured himself in Arkham City, Batman was still infected with Joker's blood by the events of Arkham Knight — considering that Batman only took half a dose of the cure, and intended to give Joker the other half, it could be assumed it was not enough to completely cure him in the long run and was only intended to save him and Joker from immediate death to allow curing later. It's also possible that the particular sample of Joker's blood that was injected into Batman couldn't be cured along with the Titan Disease because the Joker Infection was a result of a mutation and was not really a poison like the Titan Disease.
- It's not clear whether or not Joker knew of the effects of his blood. While the hallucinated Joker is aware of the effects and implies that he knew it all along, it doesn't confirm that the real Joker knew of them. In addition, Harley shows no signs of the blood's effects, thus meaning that Joker either didn't know of the disease or didn't trust her sanity enough to infect her with his blood.
- Similarly, it was also not made clear whether the various hallucinations of the Joker were hallucinations or his actual ghost, although some of the scenes (most notably Batman flashing back to the events of Jason Todd's torture and "death" as well as the events of Barbara Gordon's crippling, despite him not being directly present to witness these events) imply that those infected, or at the very least Batman, managed to at least gain the Joker's memories.
- Its possible that this disease helped inspire an evil alternate version of Batman in the comics known as The Batman Who Laughs. A Batman who killed the Joker and was subsequently infected by a toxin from within the Joker's body, he then failed to resist the infection and became a Jokerized version of Batman who went on to sadistically jokerize and slaughter countless people, including his family and former allies.
- There is however one notable difference, the Joker infection appears to largely suppress who the victim's original identity was and effectively turn them into Joker copies, and while The Batman Who Laughs does inherit many of the Joker's personality traits he never actually considers himself to be the Joker.
- The Joker Infection as well as it gradually stripping Batman of his restraints was given an indirect allusion in the ending of the Gunrunner Most Wanted mission, where Batman, after he subtly threatened to kill Penguin by having him choose between the GCPD or the local morgue after the latter referenced his capture of Nightwing, admitted when Penguin reminded him of his rule against killing that the Caped Crusader "wasn't feeling [himself] that night", which implies that the Joker Infection had it lasted long enough would ultimately force Batman to break his one rule. It should be noted that this exchange only happens if the player hadn't encountered Scarecrow yet upon completing the mission.