Retroviruses are one of the most widespread and probably the most biologically diverse group of infectious agents of vertebrates. Virtually all mammals—as well as some birds, reptiles, and fish—have yielded infectious retroviruses when examined sufficiently closely. Within a species, the viruses isolated can display considerable diversity of biological properties. For example, within the group of closely related murine leukemia viruses are found agents which differ in receptors used for infection; in mode of transmission for genetic (as endogenous germline proviruses) to horizontal and vertical (via milk or infection in utero); in pathogenicity from benign to highly virulent; and in disease spectrum from a variety of malignancies of varying latency to immunodeficiencies, anemias, neurological diseases, and others.