Great tits (Parus major) are small hole-nesting passerines that are heavily infested with ectoparasitic hen fleas (Ceratophyllus gallinae). The ease by which both the hosts and their parasites can be manipulated in the wild makes them the ideal model system to experimentally investigate host-parasite coevolution under natural conditions. In this talk, I will provide an overview of my PhD work at the University of Bern, Switzerland, in which I investigated the short- and long-term consequences of a parasite infestation early in the host's life. I will discuss behavioural, physiological and immunological defence mechanisms, as well as the costs and benefits of these parasite-induced host responses. More in general, I will address the importance of transgenerational phenotypic plasticity for coevolutionary processes in host-parasite systems.