When a new pathogen emerges, antibodies to the new agent only become detectable in IVIG preparations after a certain proportion of plasma donors have contracted the infection and successfully recovered from it. An even higher number of convalescent plasma donors is needed to result in neutralizing antibody (nAb) levels high enough for the resulting IVIG to afford protection against the new infectious agent. After the arrival of West Nile virus (WNV) in the United States, for example, it took several years before the prevalence of WNV nAbs reached approximately 0.5% in the plasma donor community, at which point they became detectable in IVIG lots produced from plasma donated in the United States. Thereafter, a significant proportion of IVIG lots even reached in vivo protective levels [1].