High host density, stressful thermal (Bruno et al., 2007) or oxygen regimes (Aeby and Santavy, 2006) , and even mating (Rolff and Siva-Jothy, 2002) are some natural situations that can also lower immune competence. To this list can be added a number of human-imposed immune stressors including crowded aquaculture conditions (Flemming et al., 2007) , use of harmful chemicals on fields or roads (Rohr et al., 2008; Karraker and Ruthig, 2009) , altered diets (Sahu et al., 2008) , elevated or altered environmental nutrient conditions (McKenzie and Townsend, 2007; Wedekind et al., 2010) , and deliberate implementation of immunosuppressive therapies. Not only can these situations lower host immune competence, they may also increase parasite virulence and thereby alter probabilities of successful colonization in a new host species (Wedekind et al., 2010) .