Parasites can influence different host behaviours including foraging, mate choice and predator avoidance. Several recent papers have shown reduced learning abilities in infected insects. However, it is difficult to separate the effects of the immune response from the direct effects of the parasite. Using a free-flying learning paradigm, this paper shows that learning performance is impaired in bumble-bees (Bombus terrestris) that are not infected but whose immune system is stimulated non-pathogenically. This demonstrates that before it is assumed that a parasite has a direct effect on a host's behaviour, the effect of the immune response stimulated by the parasite must first be quantified.