Significance Interactions between pathogens can have substantial negative effects on host health. Therefore, identifying the causes of these interactions is integral for effective disease management and control. Interactions between two pathogens that simultaneously or sequentially infect the same host are common. We show that interactions also arise when genetically based defenses against one pathogen affect responses to another, even when the first pathogen is concurrently absent. This mode of pathogen interaction provides insight for understanding patterns of variation in disease severity in natural populations. It also implies that, under some conditions, control strategies that rely on selecting for resistance to one pathogen, in livestock for example, might jeopardize control of another.