Most research on Internet auction fraud focuses exclusively on the relationship between the con-artist and victim. However, the con-artist and victim are situated in an ecology comprising the auction house, police, and auction community. This paper employs the 'parasite' metaphor as a way of building theory about Internet auction fraud. We begin by describing the parasite metaphor. We then introduce three theories from the parasitism literature and demonstrate the insights these theories can produce. The first theory, the competitive exclusion principle, highlights how separate auction markets evolve their own species or types of fraud. It also warns us that fraud elimination may be neither desirable nor feasible relative to constraining fraud to acceptable levels. The second theory details various parasite infection mechanisms to show that on-line fraud is composed of two processes; the actual deception and escape. Finally, virulence theory provides one way to predict how much harm a particular kind of fraud will cause to an individual victim. Virulence theory is also used to suggest that the auction infrastructure encourages low virulence vis-a-vis other kinds of fraud like Nigerian letter fraud.