While such masses could be inflammation or tumour of many different sorts, the vast majority in domestic rabbits are associated with myxomatosis. The relation between the myxoma virus and the host rabbit is a fascinating story (very well reviewed in Zuniga MC: A pox on thee! Manipulation of the host immune system by myxoma virus and implications for viral-host co-adaptation. Virus Res. 2002 88:17-33) with viral virulence factors and host resistance in a major host-virus battle. This rabbit had been vaccinated and thus was not as severely affected as an unvaccinated animal such as this poor rabbit.