Retroviruses are documented from different host species worldwide. Retroviruses show host specificity, the specificity being determined mainly by the presence of viral receptors on the host cell surfaces. Retroviruses can be transmitted in two ways: exogenous and endogenous.
· Most oncogenic retroviruses are exogenous. They are trans-mitted horizontally.
· Endogenous retroviruses are transmitted vertically from parents to offspring, and the process is mediated by the provirus integrated with germ line cell genome. This provirus behaves like a cellular gene and is under regulatory control of the host cell.