PZ Myers wrote a post that brings together many convergent threads. I had heard of parasitic twins and of teratomas before, though I was unaware of the medical term fetus in fetu.
Fetus in fetu can arise in one of two ways: an identical twin never fully develops, and instead is (literally!) incorporated into the surviving twin, OR, a teratoma (germ cell tumor, from the Greek word τέρας, "monster") forms, and, because of the nature of germ cells, differentiates into developed tissues. The effect is quite freaky--I have seen a picture (I think in Teratologies, by Jackie Stacey; I can verify that when I unpack my books) of a tumor with some teeth and hair--very disturbing!
I suspect that one reason it is so disturbing is that this kind of teratology forces us to confront how similar reproduction, aging, cancer, and death are at the cellular level. Normally, we think of these as very different from one another, divided by reasonably clear boundaries. Teratology trangresses, literally "steps across" these boundaries, and brings forward things we normally don't face.
I'm just guessing now, but I think this could account for something I observed while I was practicing massage more actively--some of the most touch-deprived patients I encountered were either terminally ill, had cancer, or both. I think that normally we can draw a mental boundary between cancer or death and us "healthy people"--and being confronted with those conditions makes that conceptual boundary harder to maintain. Once again, a "transgression" has taken place. In the rational, logical part of our brains, we know that neither cancer nor death is contagious, but unconsciously, we shrink from touching. As a result, a normal human need that we Americans are short on anyway becomes even more scarce for a population which, paradoxically, needs it perhaps more than ever.
I don't think it is wrong to have these feelings; it is perfectly understandable. But to take the extra step to get past them--to decide to acknowledge their uncomfortableness and reject them in favor of reaching out to someone--for me, that is a true mark of insight and kindness.