"Big `C'"

by Brian Lumley
Originally published Lovecraft's Legacy, 1990

The discovery of a second moon in orbit around earth, Luna II, in 2013 leads to a manned expedition, its star - Benjamin "Smiler" Williams.  Who happens to have terminal cancer.

Williams returns, changed.  He still has cancer. But he's not dying of it.  And then "Big C", as Williams calls his illness, decides to exit his body (we are mercifully given only a brief description of this).  And, as a sentient being, it (a) takes over the space installation where Williams is housed (b) gets its tentacles on a bunch of nukes and (c) claims a big chunk of Florida as its own.

Peter, Williams' old friend, is sent to visit Williams and (surreptitiously) detonate a nuclear device to kill "Big C".  It turns out Williams is long dead, merely animated by "Big C" who has drained his memories and such.  "Big C" has also infected Peter (the reason he was sent on the suicide mission).  And he's taking over everything.

Lumley claims this is a sequel of sorts to "The Colour Out Of Space", though I myself can't see it.  Conceptually, it does sort of recall the original Quatermass, though.

In any case, this is decent Lumley tale that's almost amusing in its nastiness.  He really seems to have been having a good time with its grimness and grue.