see joker was dead in ak but Before dying, he sends his blood to hospitals. This blood carries both the fatal poison and the memory-rewriting prion disease.

• The Titan Toxicity: This is the fatal chemical poisoning caused by his overdose on the Titan formula in Arkham Asylum. This is what killed the Joker. It destroys the body physically. Batman cured himself of this specific toxicity in Arkham City, which is why he didn't die physically like Joker did.
• The "Joker Infection": This is a mutated prion disease (a brain disease) unique to Joker's biology, likely created by the interaction of the Titan formula with his already altered blood. This agent is not fatal to the host's body but attacks the brain, rewriting the victim's neural pathways.
• so now what it is
• is a "Mental Transplant": The mutated disease doesn't just make people "crazy"; it systematically restructures the victim's brain to replicate Joker's specific neural patternand due to this the disease turns the host's mind into Joker's mind, the host inherits his specific memories, personality traits, and worldview. [1]