Updated.
Disease or cursed wound (like a clay golem)?
The references to infection suggests disease to me, especially as there's no mention of
remove curse. The prevention of mundane healing suggests a supernatural disease, like a "Magical Curse disease", like a less nasty form of Mummy Rot.
There's no mention of it causing any other deleterious effects, but I think we should have it do some ability damage. Maybe base it on Filth Fever?
Filth fever—Injury, Fortitude DC 12, incubation period 1d3 days, damage 1d3 Dex and 1d3 Con.
Maybe something like:
Siren Rot (Su)
Supernatural disease—claw, Fortitude DC 15, incubation period 1 minute; damage 1d3 Dex and 1d3 Con. The save DC is Charisma-based.
A creature suffering from siren rot can not recover hit points or ability damage points from natural healing, but magical healing spells function normally.
Siren rot is part curse, part natural disease. Unlike normal diseases, siren rot continues until the victim reaches Constitution 0 (and dies) or is cured as described below.
A
remove disease spell cures siren rot. A
break enchantment or
remove curse spell neutralizes the curse portion of siren rot, leaving the victim infected with the normal disease, which can then be healed naturally or magically. The mundane portion of siren rot is filth fever:
Filth fever disease—Fortitude DC 12, incubation period (already incubated); damage 1d3 Dex and 1d3 Con.