The majority of ERVs that occur in vertebrate genomes are ancient, inactivated by mutation, and have reached genetic fixation in their host species. For these reasons, they are extremely unlikely to have negative effects on their hosts except under unusual circumstances. Nevertheless, it is clear from studies in birds and non-human mammal species including mice, cats and koalas, that younger (i.e., more recently integrated) ERVs can be associated with disease.  The number of active ERVs in the genome of mammals is negatively related to their body size, suggesting a contribution to Peto's paradox through cancer pathogenesis.  This has led researchers to propose a role for ERVs in several forms of human cancer and autoimmune disease, although conclusive evidence is lacking.