Biological invasions often have negative impacts on native biota. This is particularly true if the invasive species is a parasite. Blue mussels Mytilus edulis in the North Sea were invaded as a new host of the parasitic copepod Mytilicola intestinalis in the 1930ies starting a new coevolutionary arms race. Here, we explore the evolution of parasite and host traits along separate fronts of the invasion and how infection with the new parasite affects host physiology directly. However, next to direct effects, this host-parasite interaction can also have profound indirect effects that feed back on mussel fitness. These include changes of gut microbiota, resistance to secondary infections but also the interactions with epibionts as well as predators indicating that indirect effects can outweigh the direct effects.