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.
Don't u see that's a joke. (=・ω・=)
It's love sickness, I presume! (๑•ㅂ•)و✧ jk.