Centuries of practice clearly demonstrate that immunization by means of vaccination reduces the disease burden in human and nonhuman animal species. While the emphasis on individual health and fitness is important, the role of immunization in maintaining the health of entire populations or communities is also important. I developed a simulation that allows participants to actively explore the value of herd immunity in controlling the spread of infectious disease, first by considering how quickly an entire population may become ill if no one has immunity, and then after immunization of 80% of the population. Pretest, posttest, and four-week follow-up questionnaire data collected from nonmajor introductory biology students suggest that active engagement in an activity such as the one described may contribute to longer-term retention of the principles and concepts than presentation by lecture alone.