The COVID-19 pandemic has forced children to reckon with the causal relations underlying disease transmission. What are children's theories of how COVID-19 is transmitted? And how do they understand the relation between COVID-19 susceptibility and the need for disease-mitigating behavior? We asked these questions in the context of children's beliefs about supernatural beings, like Santa and the Tooth Fairy. Because these beings cannot be observed, children's beliefs about the impact of COVID-19 on them must be based on their underlying theories of disease transmission and prevention rather than on experience. In the summer of 2020, N = 218 U.S. children between the ages of 3 and 10 years (M = 81.2 months) were asked to rate supernatural beings' susceptibility to COVID-19, and the extent to which these beings should engage in disease-mitigating behaviors, such as social distancing and mask wearing. Many children believed supernatural beings were susceptible to COVID-19. However, children rated the need for supernatural beings to engage in disease-mitigating behaviors as higher than the beings' disease susceptibility, indicating a disconnect between their conceptions of the causal relations between disease-mitigating behavior and disease prevention. Children's belief that a particular supernatural being could be impacted by COVID-19 was best predicted by the number of human-like properties they attributed to it, regardless of the child's age. Together, these findings suggest that although young children fail to appreciate specific pathways of disease transmission, they nonetheless understand disease as a bodily affliction, even for beings whose bodies have never been observed.