DescentBB

What's your theory about where the alien virus came from?

Personally I think it was created by an alien race from some other planet. Then some idiot comes along (maybe Dravis, maybe not - depends on which mission still in the planning phase you ask about - though Dravis does wind up acquiring it).

As for why the alien race created it? As a weapon against another alien foe and/or rebels among their own people. Whichever person found it realized that it could work in other scenarios (i.e. for whatever reason it was used on PTMC).

I always liked to imagine that the organism evolved naturally. Perhaps it was discovered in an unknown mine and, after seeing the potential to reduce manufacturing and mining costs, the PTMC tried to control an manipulate it. The virus escaped and rapidly followed supply routes to other mines, causing the PTMC to panic as it crept ever closer to Earth.

I came up with one idea that was going to be used in the Sol Contingency game... don't know if it still is, I've been left out of the loop there for a while now.
It was quite clever -- didn't entirely match up with the previous continuity but that's what reboots are for. I'll expound when I'm positive I won't be spoiling anything.

One other idea I had was that it was a disembodied alien intelligence trying to figure out how corporeal life works by observing humans through their machines (sending parts of itself to Sol as nanotech to subvert those machines to its purpose), and isn't so much actively trying to destroy humans as just... not having a very gentle touch when it pokes and prods. It doesn't much care if it hurts someone, or if it wipes out a civilisation or three.
It used Tycho Brahe, the Mercenary planetoid, and four other planetoid-like objects as "listening posts" on this solar system and others, but after Dravis tried to subvert the virus and found out what it really was he was terrified of it; and even after gaining control over a portion of it he still had at least three of its planetoids (Tycho Brahe, the bees one, and Charon, and possibly one other), either destroyed or rendered unusable before his death.
Two of the other three were undiscovered by anyone on Earth at the point when the main games took place, and nobody but Dravis had any idea that the last one was anomalous at all.
This was a backstory for another big solo project I started and never finished, which picked up where Mercenary left off and went on till sometime after the ending of Descent 3. Much like the other one, I had it very well thought out. An interesting villain makes an interesting story after all.