Nope, you're already infected.
The guy who gave you the tenebrium chest mentions that you can get cured at the church in his village, I haven't tried that but it might work. You'll also get cured during the Immaculate trial, so if your level is high enough (the enemies are lv 14 but there's not a lot of them) you could do that.
You also don't need to defeat the troll king to get the immunity, there's some Tenebrium ore in the Luculla mines you can grab and hand over.
