Scale Rot

A Regular Fish Disease

Not an uncommon disease around the known world, scale rot includes a handful of regional variants. It is a disease that affects fish, specifically their scales. Slowly, over a period of about a month, the scales start to decay, leaving patches of bare skin. This exposure makes the fish more susceptible to contracting other diseases. The more aggressive variants of scale rot also attack the bone.
In general, ocean fish are relatively safe from the disease due to the endless space decreasing the chances of coming into contact with diseased fish. Secluded waters such as lakes and ponds pose a greater threat. It can occur that entire fish populations die out, but this is rare.

Extreme Danger for Merfolk

Two extremely dangerous zoonotic variants of scale rot have been recorded, both in the Sparkling Chain. Here, scale rot spilled over from a species of surgeonfish onto the Inu Zim (the local merfolk population) twice, about a hundred years apart. In both cases the infection rate between merfolk was far greater than that between fish, and the diseases affected both scale and bone. Many merfolk succumbed to the diseases, but especially during the first outbreak. During the second outbreak an accidental cure was found.
The Inu Zim lead an amphibious lifestyle, but spend way more time beneath the waves than on land. It was an Inu Zim botanist (a student of the famous Enthuri turned merfolk botanist Adelard Greenhand) suffering from the beginning stages of scale rot who was on a land expedition, and stumbled upon an extremely gross and putrid fruit. Literally. He stumbled down a cliff and stepped straight into the massive cucurlen. Being far inland on one of the bigger islands, it took a while before he reached water to wash his legs. The next day he noticed his scales to be less inflamed.