Thinking about how aside from the skin decay/bruising, Leon’s t-virus infection pre-re9 would manifest in an enthusiastic oral fixation with heavy interest in biting. He’s always had an oral fixation, but previous to the infection truly settling in preferred being bitten to being the biter.
Over time that shifts a bit. Kisses to your shoulder start being led by a gentle nip that’s then soothed by the press of his lips to that spot. What once was a single nip/kiss combo became a trail of gentle bites soothed by his mouth — the true benefit being that you’re just as turned on by those actions as he was. Your skin has a particular taste that he’s grown to enjoy, soon he’s requesting you not use heavily scented lotions because they alter that taste and the more natural scent you have is significantly more arousing than the cutesy scents (but he did once love those too, his taste has just shifted).
Your skin becomes littered in his bite marks, mostly able to be covered but there were some suspicious bruises that peek out from under your clothes. Do either of you understand why he’s suddenly developed this desire? No, but you weren’t complaining about this more affectionate side of him. The sex also became much more involved, as if a more feral side of Leon was making itself known and pushing you both to heights previously unknown with the unique mix of pleasure and pain made possible by his new biting habit.