You likely just started in a chunk with naturally high Taint levels--the teleportation seal was likely the last straw that pushed things to tip over.
If you've got boatloads (READ: three or four FULL alchemical bags/chests' worth) of cobble and/or wood planks, you could try to wall off the Taint to prevent it from spreading...make sure you get it all underwater, and make the wall 2 or 3 blocks thick when you're on a slope--ANY exposed surface of a block touching Taint could allow the infection to "jump" the barrier. Otherwise, I'd advise you to pack up your stuff and relocate--preferably somewhere far enough away that the Tainted chunks will unload from memory, as Taint can't spread in unloaded chunks. ;)