Second, an inescapable feature of the modern world is the frequency with which invasions of exotic species occur (Torchin et al., 2002) . Invasive species might be either hosts or parasites (possibly including their vectors), and all can present dire and often unpredictable consequences, including extinction, for indigenous organisms (Wyatt et al., 2008) . A role for immunology in influencing invasions can occur in at least three ways: