When I first learned that mitochondria (and chloroplasts) have their own DNA and their own personal DNA replication mechanism, I wondered if they had their own viruses that hijacked that replication mechanism. It was a long time coming, but it seems that there are indeed viruses that infect the mitochondria of some fungi. Obviously, this kind of thing is particularly likely in fungi.
Since living cells are exchanged between mother and unborn children, and persist for decades, there is a possible niche for cells that are transmitted from mother to daughter, and on to granddaughters after that, and so on. Probably to sons as well, but they would most likely be dead ends. It is easy to show that a maternally transmitted host cell line would have to have effects on fitness that are no worse than neutral – in practice, beneficial.
So humans could well carry a maternally transmitted symbiote that was originally human – something like canine venereal sarcoma.
This ought to be true of mammals generally, and even if we don’t have one, some other mammalian species might.