It will live in the sand/rock, find a host (fish) and continue on. The only two proven cures are Hyposalinity or copper treatments. Those are done in a QT. You leave your DT fallow for 6-8 weeks to let the Ich in there die off due to not having a host. By then, your QT fish should be cured and will go back into an Ich-free tank. QT your new fish moving forward and you should never have Ich in your DT again.
It really stinks when you almost have your tank stocked and then a new fish is introduced with Ich because you were not QTing. You could lose a lot of fish, or you end up pulling them all out into QT. Doing it right from the start usually saves lots of heart/headaches down the road.