Over the last 40 years, disease outbreaks have significantly reduced coral populations throughout the Caribbean. Most coral-disease models assume that coral diseases are contagious and that pathogens are transmitted from infected to susceptible hosts. However, this assumption has not been rigorously tested. We used spatial epidemiology to examine disease clustering, at scales ranging from meters to tens of kilometers, to determine whether three of the most common Caribbean coral diseases, (i) yellow-band disease, (ii) dark-spot syndrome, and (iii) white-plague disease, were spatially clustered. For all three diseases, we found no consistent evidence of disease clustering and, therefore, these diseases did not follow a contagious-disease model. We suggest that the expression of some coral diseases is instead a two-step process. First, environmental thresholds are exceeded. Second, these environmental conditions either weaken the corals, which are then more susceptible to infection, or the conditions increase the virulence or abundance of pathogens. Exceeding such environmental thresholds will most likely become increasingly common in rapidly warming oceans, leading to more frequent coral-disease outbreaks.