  We show in a model-independent way that, in the background of a topological
soliton or instanton that saturates a Bogomol'nyi bound, the fermion and boson
excitation spectra of non-zero modes cancel at the one-loop level. This
generalizes D'Adda and DiVecchia's result for some specific instanton models.
Our method also establishes, again in a model-independent way, the generality
of the connection between zero modes in topologically non-trivial backgrounds
and index theorems.
