It consists of exogenous horizontally transmitted viruses found in several groups of mammals.

[1] Infection is usually asymptomatic, but inflammation and cancer can develop over time.

[1] Known exogenous deltaretroviruses infect cattle and primates.

[1] The two complete endogenous ones were found in bats and dolphins; the others in Solenodon, mongoose, and fossa.

These endogenous examples fill in the large gap in the host range.