In Descent 2, the "alien" virus turns out not to have been alien in origin after all.
The novels feature a story more in line with the above, having the virus indeed be alien in origin, with Dravis, who is a much more of a Magnificent Bastard in the books, instead stealing it and altering it for his own purposes in the third novel.
Descent 3: Mercenary sets the record straight by revealing that the PTMC's encounter with the virus was indeed by chance. The missions in the singleplayer Mercenary campaign take place during the period when Dravis attempts to control the virus (this is after the end of Descent II), and finally succeeds in doing so, using the SRAD Research Laboratory and CED outpost on Io as testing grounds for the controlled strain.
Descent 3, it seems, was originally planned to be much more complex than what it is now. An old video on what used to be PlanetDescent shows a short segment of Level 3 when Descent 3 was still in testing, and an ancient Adrenaline Vault feature used to detail every single robot's strengths, weaknesses and capabilities, such as the Juggernaut being able to kneel down to dodge a missile, the EMD Gun being the only weapon capable of short-circuiting the Spy Hunter's self-destruct mechanism, or the Thief being able to generate three decoys to fool a pursuing player. Traces of some of these additional features are still present in the final game (like the Stinger being more prone to friendly-fire incidents, or the EMD Gun being particularly effective on the Spy Hunter). Some levels, such as Level 6 and 15, were originally meant to have much more eventful boss battles, such as the player navigating a maze while being chased by a Homunculus shrouded in darkness, or the Hellion Assault Mech smashing out huge sections of the cavern he is in to generate lava falls. In addition, Level 3 of Mercenary was originally conceived such that the player would have a more drawn-out battle with the Tracking Station's Captain; he would remove himself from the lavatory and enter a secret door to the side into a very small room. Descent 3 was also wildly anticipated by fans of the franchise, and had Interplay invested their funds in the right kind of advertising (a tournament held in the US is not the right kind of advertising), Descent 3 may have had a larger, albeit very broken, fanbase. Further details may be found here. You might be surprised at the wealth of information over there.
Descent 4. A fourth title in the series got pushed back when the developers Parallax split into two separate companies, Outrage and Volition, of which Volition made a sort-of sequel in the form of Descent: FreeSpace, and a lot of rumoured elements of Descent 4, such as the character's name (Parker) got recycled into the then new series, Red Faction.