Essentially it does long term damage and a few aches and pains here in there. I was currently going to have the nurse say it was a heart disease then go out and tell Higoroshi that he was infected.
The parasite will considerably drain Kakusu of his energy and he will be generally weaker than other Youko his age (in fact, I wanted it to be why he was weaker). I also wanted to say that he can draw energy from the parasite in the long run to fight normally although it might end up in a few permanently damaged limbs or organs possibly even death, if used consecutively in a fight or something.
Maybe even give him a sickly appearance and so on. The parasite isn't going to make him fight powerfully and essentially it's going to be a limit on his fighting ability unless he draws on it's powers, which will only enable him to fight at his normal strength for a certain amount of time.