Our work investigates the annual outbreak of Whypox, a virtual epidemic in Whyville.net, a virtual world with over 1.2 million registered players ages 8-16. We examined online and classroom participants' understanding of a computer virus using surveys and design activities. Our analyses reveal that students have a mostly naive understanding of a computer virus influenced by mythological or anthropomorphic perspectives; only few were able to describe computational elements. The 35 students who participated in a curricular intervention in addition to the virtual epidemic shared these naive conceptions initially, but developed more sophisticated views after the intervention. The discussion addresses possible explanations for students' conceptions and implications for future instructional designs.