I'm sure everyone knows by now that there are spoilers, so here I go...

Bahamut's fayth talked to Tidus, saying that "they (the fayth) were watching him for a while". The Fayth dreamed up Jecht and Tidus both... with a desire to bring them into Spira. My guess is that the fayth who became Sin before Jecht talked to the fayth and was convinced to bring Jecht here, so that Spira's suffering would end. Unfortunately, Jecht chose to become the final aeon... why? Auron and Braska asked Jecht why, and Jecht's reply was about a plan and he laughed...

Bahamut's fayth, as I mentioned, tells Tidus that the fayth were watching him and Jecht... what I believe he meant was this: they dreamed up these two in order to destroy Sin permanently.

And the "toxin" thing was mostly a lie forged by Yevon, I think. Although being close to Sin did effect some people around Spira, but I think it's mostly psychological.