Immune systems maintain the integrity of organisms by recognising and attacking foreign substances and/or pathogens. However, immune defences can only take place following direct contact with threats. Disgust can prevent infection before contact with potential pathogens: we propose that disgust is an evolved nervous response to a signal reliably co-occurring with infectious environmental disease threats, which motivates behaviour leading to the avoidance of infection. We hypothesize that disgust and immunity form a defensive continuum with overlaps: disgust acts prior to contact with the infectious agent and prevents it from getting into the body; emesis (vomiting) gets it out once inside the gastrointestinal tract, before penetration of the body boundaries; and immunity expels or kills infectious threats following penetration of the body proper. We further propose that serotonin (5-hydroxytryptamine, 5-HT) might be the link between disgust and immunity. 5-HT plays a central role in the induction of the emetic reflex and is possibly involved in the development of learned aversion; it is also a signal used by immune cells and modulates both innate and acquired immunity. We therefore propose 5-HT might mediate the interaction between these two defensive mechanisms.