In the final episode of the anime, Nana and Hachi (who used to be parasites in the past) claim that they're immortal:
We don't age and we're stuck in the middle, neither adults nor children. We have but one duty: to reach out to humanity's future and continue to watch over it. ~Hachi
But just a few minutes later the conversation between Futoshi and Ikuno seems to contradict it:
It's all thanks to your research that us ex-parasites' accelerated aging is being kept in check. ~Futoshi
What information have I missed? Are ex-parasites immortal or mortal?