He does not seems to be bothered by drugs and diseases, and resilient enough to have a degree of regenerative abilities, albeit slow.
The Leper's main power is the ability to shortly accelerate development of any pathogens in his victim's body. The effects vary from short incapacitation to being lethal, specifically when paired with a certain Syringe-Breaker.
↑A hulking form lunged into view, coming within a handspan of getting a grip on Precipice. Copper mask, partial armor, and a loincloth, and a body covered in oozing sores, blisters, and scabs. Copper chains wound around his arms, and swollen, infected hands gripped the hooks at the end of those chains.
He was big and fast enough that he didn’t stop by his own power. Instead, he hit enough of the outcroppings of needles that he was impaled sufficient times that they made him stop. - Excerpt from Dying 15.4
↑Sure enough, the Leper was recovering. A hole yawned in the center of the vaguely head shaped mass of congealed human sickness, the beginnings of a mouth. I could see nuggets that might have been congealed pus or nascent teeth. - Excerpt from Dying 15.4
↑“He used it on them too,” he said, pointing at the three dead. “It triggers every latent disease in your system for a few seconds of effect. They got pricked, scraped, or injected by needles before he used his power.” - Excerpt from Dying 15.4
↑“The thralls called him the Leper. He killed four of ours,” Gong said. “We can’t let him heal, we can’t bring him with, and we need to move. It would be best if you ended this now. Getting to you and getting back would take too long. We need to help other groups. All of us are struggling.” - Excerpt from Dying 15.4