Koch's postulates are regularly included in the lecture portion of microbiology courses, but rarely are they demonstrated in a microbiology teaching lab. This is understandable given the logistical challenges of undergraduates working with pathogenic bacteria, ethical concerns using animals, and limited time constraints of a weekly lab period. Here we present a cost-effective, timefriendly lab activity that demonstrates the principles of microbial isolation and infection assays that are part of fulfilling Koch's postulates. The disease is “peep pox” caused by a gelatinase-positive bacterial species hydrolyzing marshmallow peeps that proxy as infected animals.