The first stages of the plague the astronauts brought back from Enceladus were terrifying enough - it started as a sticky, brown-grey paste that slowly accumulated between your fingers and toes, but then it would suddenly explode, spreading to cover the whole body in a thick mucus.
As horrifying as that was, the second stage was even worse, as the mucus slowly hardened into a crisp, papery coating as though the victim had been encased in a cocoon.
The first efforts to reverse the process surgically revealed the victims within at various stages of dissolution, their bodies apparently crumbling into a slurry. It was widely conceded that the Enceladus Syndrome was fatal.
Until the first cocoons started to open of their own volition...
That was when we realised: it wasn't a plague.
It was an invasion.