  We derive a general obstruction to the existence of Riemannian metrics of
positive scalar curvature on closed spin manifolds in terms of hypersurfaces of
codimension two. The proof is based on coarse index theory for Dirac operators
that are twisted with Hilbert C*-module bundles.
  Along the way we give a complete and self-contained proof that the minimal
closure of a Dirac type operator twisted with a Hilbert C*-module bundle on a
complete Riemannian manifold is a regular and self-adjoint operator on the
Hilbert C*-module of L2-sections of this bundle.
  Moreover, we give a new proof of Roe's vanishing theorem for the coarse index
of the Dirac operator on a complete Riemannian manifold whose scalar curvature
is uniformly positive outside of a compact subset. This proof immediately
generalizes to Dirac operators twisted with Hilbert C*-module bundles.
