From evolutionary reasoning, we derived a novel hypothesis that ingroup derogation is an evolved response of behavioral immune system which follows the smoke detector principle and the functional flexibility principle. This hypothesis was tested and supported across three experiments. In Experiment 1, participants' group membership was manipulated by using a minimal group paradigm. The results indicated that mere social categorization alone - a heuristic cue that implies the differentiation between "us" and "them" - was sufficient to elicit ingroup derogation among Chinese participants, and, such an intergroup bias was positively associated with the perceived vulnerability to diseases, which was also more consistently associated with ingroup attitudes. Experiment 2 extended and partially replicated Experiment 1 by showing that when there were cues of diseases in the immediate physical environment, Chinese participants exaggerated their attitudes of ingroup derogation. The results also showed that this effect was mainly driven by outgroup attraction. Experiment 3 changed the method of disease manipulation, and found that Chinese participants responded more strongly to disease cues originating from ingroup members and that they endorsed more ingroup derogation attitudes even when the ingroup and outgroup members were both displaying cues of diseases. Taken together, these results reveal the previously unexplored effects of infectious diseases on ingroup derogation attitudes, and suggest an interesting linkage between the evolved behavioral immune system and the ingroup derogation.