Turtles with FP are found to have a compromised immune system. They have higher phagocytic leucocyte counts (especially heterophils) compared to healthy individuals, which seems to be an effect of FP, as it is mostly evident in individuals with severe tumours. This further supports the hypothesis of the herpesvirus as a causative agent. Immunosuppression is strongly correlated with FP, but does seem to be a consequence of the development and growth of FP rather than a prerequisite, which is similar to other virus-induced tumour diseases in other species, such as Marek's disease in poultry.