The Joker ailment was a mystical infection featured in Persona 2: Eternal Punishment.
Nyarlathotep, in its bid to force the restoration of the Innocent Sin timeline during the events of Eternal Punishment, arranged for his servant Tatsuya Sudou to "become" a second iteration of Joker, JOKER, with a similar modus operandi: having a person call themselves on their cell phone and have JOKER appear, though instead of openly granting any wishes, JOKER asked who they desired to see dead.
Many people were seduced by the promise of power and called. However, as the New World Order invaded the airwaves to broadcast a message through fortune-teller Chizuru Ishigami to spread an insidious rumor: anyone who had ever used the services of JOKER will become a Joker themselves. Within hours, dozens of Sumaru City residents were transformed into Jokers. People who had called the JOKER began losing their minds, and when confronted, they became giggling lunatics, and their faces lost all definition except for a warped smile reminiscent of Joker's. In this state, they became dangerous powerhouses, and gained the ability to summon a Persona-like beast calling back to the original Joker, which gradually degraded to a vicious monster only partially resembling its original shape.
During Eriko's route to Sumaru TV, the party witnesses Ishigami's further extension of the Joker curse to anyone who had never called the JOKER. The whole scheme of turning more people into Jokers involves the viewing of the Joker as a symbol of sin (due to the contract killing). If more people feel sinful because of the Joker Curse, the New World Order can lure them to their seminars to absorb their Kegare (Japanese archaic form of "filth"). The large quantity of Kegare would force the dragons underneath the earth to rise, ultimately destroying the whole world. These events are, of course, rumors spread by the New World Order under Nyarlathotep's influence.
Jokers, besides having the standard summoning and combat abilities, have the Old Maid ailment skill, which will temporarily turn a random enemy into Joker, making them lose control of themselves and attack their allies.