This study examined students' perceptions of the risks associated with microbial transmission before and after taking a microbiology class. Participants included undergraduate students (n = 132) enrolled in a microbiology course at two universities and one community college. Students completed a survey at the beginning and end of the course and a sub-sample of students also participated in pre- and post-course interviews. The survey results showed that there were changes in students' knowledge of microbes and microbial transmission as well as changes in reported behaviors related to microbial transmission. The behavior changes primarily involved perceptions of microbial transmission via touch such as using a public telephone, touching a faucet in a public bathroom, handling money, borrowing soap from a friend while camping, working in the dirt without gloves, and taking out the trash. There were no differences in behaviors related to oral transmission (eating, drinking, and kissing) or for preventative behavior (use of sanitizing items or frequency of hand washing). Exploratory factor analysis showed a subtle shift in the conceptualization of transmission risks from pre- to post-course. Pre- and post-interviews revealed that although students gained a more sophisticated concept of the positive role of microbes in the ecosystem, they were not more likely to report changing their behaviors related to vaccines to avoid getting sick from pathogenic microbes.