Pathfinder 2e General Discussion

The immediately obvious way to make such a disease is to make it an actual disease using the affliction rules. This however would get easier to deal with as your characters level up, since the DC will stay constant but the PCs saves will go up. Another way to go about it would be to have a lost of conditions, which accumulate for each day/week. So on the first day/week you gain Clumsy 1, then also Enfeebled 1, then also Clumsy 2 and so forth. This will very quickly become VERY punishing though, unless you carefully pick conditions that don't affect your PCs very much. The entire list of conditions can be found here. I would probably use Fatigued and Flat-footed for the first few levels and then the ones listed under Lowered abilities after that, maybe even throwing in a Doomed or two along the way. But as I said, this can very quickly become VERY punishing.