[Immortus]

Extreme spoilers, beware...^^

Spoiler for the purpose of Medusa and Kasumis sister...:

Close, but not quite: Medusa is the virus, it only befalls people with weak determination, because it was too thinly spread out in the world.

In the long run, it can incorporate the dreams of its victims, without killing the host, but Zeus uses the sister of the protagonist and turns her into an ever-dreaming monstrosity that dreams and produces monsters out of desperation.

Why ist it so desperate? Well...her sister suspects it is because she felt betrayed when the protagonist was chosen for survival. But maybe it is something different...:P

But that would be a too cruel of a revelation, even in the spoiler tag. Kind of "Sixth Sense"...