Scattered in human and animal genomes are a class of repetitious genetic elements called endogenous retroviruses (ERVs), which bear sequence homology to retroviral genomes. They constitute about 8% of the human genome. Evolutionists assume that all endogenous retroviruses are remnants of germ line infection by exogenous retroviruses. However, the essential beneficial function of some ERVs and complex interaction between ERVs and other host DNA sequences suggest that some retroviruses were created in the cell as part of the host genome. The original ERV elements were endowed with the ability to package themselves in proteins and lipids and leave the host cell as infectious particles. Thus we speculate exogenous retroviruses were derived from endogenous retroviruses (exogenization, as opposed to endogenization). The ability of retroviruses to transpose within the host cell or to infect another host may have been designed for protein-coding, for gene regulation, and for DNA repair by homologous and non-homologous recombination. After the Fall, retroviral elements have been degraded by mutations. Retroviral insertion into the host genome also became deregulated, causing insertional mutagenesis. Deregulation of exogenous retroviruses resulted in pathogenesis.