In a phenomenon known as Peto's paradox, large mammals do not develop cancer more often than small mammals, despite having more cells that could go haywire. On Life Lines, Dr. Dolittle writes "Some researchers suggested that perhaps smaller animals developed more oxidative stress as a result of having higher metabolisms. Others proposed that perhaps larger animals have more genes that suppress tumors." But a new hypothesis argues that large mammals have evolved to minimize the activity of ERVs, which are ancient viral elements integrated into our DNA. Active ERVs can cause cancer and possibly other diseases; mice exhibit about 3300 active ERVs, while humans exhibit about 350. On the blog known as ERV, Abbie Smith writes "some of the young ERVs in humans, the ones that can still code for a protein here and there, are reactivated in HIV+ patients." Researchers are considering targeting these ERVs in order to combat HIV; as Abbie writes, "You could train the HIV+ individuals immune system to ‘see’ the ERV components in an HIV infected CD4+ T-cell, and BAM! Kill the HIV infected cell!" But she warns that other ERV components are expressed in many normal human cells, and teaching our immune system to target them might be a very bad idea.