It's love sickness, I presume! (๑•ㅂ•)و✧ jk.
I doubt that it is love sickness because it was stated that he gets sick once a year for a week but the cause is unknown. The only real symptoms we are told about are paralysis and he has to stay in bed for a week that it's a condition that happens annually, this suggests to me some sort of neurological problem.
I myself think it might be a side effect of some poison he is taking himself or being poisoned by someone else. A lot of poisons effect the nervous system and can bring on a neurological disorder or problems.
We will have to wait until the author lets us know or gives us more information.










Okay. here we go at the climax. Let us prepare our hearts. ٩(๑❛ᴗ❛๑)۶