Although sociability offers many advantages, a major drawback is the increased risk of exposure to contagious pathogens, like parasites, viruses, or bacteria [1]. Social species have evolved various behavioral strategies reducing the probability of pathogen exposure [2]. In rodents, sick conspecific avoidance can be induced by olfactory cues emitted by parasitized or infected conspecifics [3–6]. The neural circuits involved in this behavior remain largely unknown. We observed that olfactory cues present in bodily products of mice in an acute inflammatory state or infected with a viral pathogen are aversive to conspecifics. We found that these chemical signals trigger neural activity in the vomeronasal system, an olfactory subsystem controlling various innate behaviors [7]. Supporting the functional relevance of these observations, we show that preference toward healthy individuals is abolished in mice with impaired vomeronasal function. These findings reveal a novel function played by the vomeronasal system.