  Various obstructions to knot concordance have been found using Casson-Gordon
invariants, higher-order Alexander polynomials, as well as von-Neumann
rho-invariants. Examples have been produced using (iterated) doubling
operations K=R(c,J), and considering these as parametrized by invariants of the
base knot J and doubling operator R. In this paper, we introduce a new mew
method to obstruct concordance. We show that infinitely many distinct
concordance classes may be constructed by varying the infecting curve c in
S^3-R. Distinct concordance classes are found even while fixing the base knot,
the doubling operator, and the order of c in the Alexander module.
