The immune system overcomes challenges by parasites and pathogens, but as disease resistance is costly and trade-offs with other life-history traits, and thus individuals have evolved disease resistance only in certain conditions when they would be most at risk. One example is at high densities when the risk of disease transmission is high. Density-dependent prophylaxis (DDP) for disease resistance is well known in insects, such as locusts, whose phenotype depends on density. In the marine realm, Echinoderms are well known for their huge population fluctuations, yet DDP for disease resistance has rare been tested outside the Arthropoda. DDP was experimentally tested in the crown of thorns seastar (COTS) Acanthaster planci, and was found for five measures of immunity spanning cellular and humoral. As COTS upregulate disease resistance during their outbreaks, it is unlikely that disease explains their mass die-offs, and renders control mechanisms more challenging. However, a widely available, free resource was tested to control COTS outbreaks: limes. Lime juice was extracted and injected into COTS, mortality measured as was any contagions on other reef organisms.