Bacteria, viruses, fungi, worms and other parasites represent a major evolutionary force that shapes host behaviors. Hosts and parasites are tightly linked in a coevolutionary process, where the first evolve strategies helping them evade infection, and the latter evolve strategies allowing them to bypass these defense mechanisms. In the first two parts of this thesis, we investigated the molecular mechanisms underlying pathogen avoidance behaviors in mice, in particular which chemosensory tools are involved in the detection of pathogen-related cues. In the last part of this thesis, we studied the host-parasite relationship of mice infected with the protozoan parasite Toxoplasma gondii. This parasite is known to alter the behavior of rodent hosts by subverting their fear of felids into attraction, thereby increasing their transmission rate to felid hosts, which are their only definitive hosts.