Mutation, diversity, natural selection and the biology of human pathogens (including antibiotic resistance) are key features of the biosciences curriculum at A Level and undergraduate study. Few resources exist to allow students to engage with these topics in an interactive manner. This paper describes an interactive, online simulation of mutation and of phase variation, a mechanism employed by many bacteria to generate genetic diversity, and includes an example of how the simulation can allow students to draw links between the genetics of pathogens and disease. Students experiment with different parameters of the simulation and use this--along with peer debate--to answer a set of questions. Use of the simulation goes some way to expel the fallacy that mutations are generated in response to environmental change. This freely available resource can be easily adapted to a wide range of audiences and topics where mutation is the underlying principle (cancer genetics for example).