The Mobius Double Reacharound Virus is a ~ATH program which Sollux wrote and shared with Karkat. This code, when executed, immediately causes the user's computer to explode and places a curse on the user forever, along with everyone he knows and everyone he'll ever meet.
Examining the source code, the program appears to split itself into two: one blue, and one red. (~ATH do us part?) It then imports two instances of the universe. What follows is inscrutable: the indentation and coloration suggests that the ~ATH blocks overlap in a manner that is entirely unlike any normal programming language.
A possible interpretation: The red code waits for the death of the first (red) universe (U1), then waits for the blue code to end. The blue code waits for the death of "!U2" (i.e., the opposite of the second (blue) universe), which has no immediately discernible meaning, then waits for the red code to end. The lines are twisted in such a way that the program can't finish executing until the universe both stops and starts existing at the same time, which apparently makes the computer explode. As Karkat puts it, Sollux is "obnoxiously good" at ~ATH.
In a fit of stupidity, Karkat runs the code, destroying his computer, cursing him and all his friends and causing the death of all but one of their Lusii.
Kanaya and Aradia believe that the virus may not actually do anything other than blowing up Karkat's computer, and that their misfortune is a consequence of playing Sgrub, as well as the fulfilment of stable time loops.
It has since been revealed that the color red is associated with the kids' universe, and blue with that of the trolls. It has also been revealed that the troll universe is the result of a Scratch, which may explain why it is referred to after the universe-importing portion as '!U2' rather than 'U2'.
This may prove to have something to do with how the kids and trolls may meet, seeing as when Beat Mesa is activated, the kids' universe will reset itself, perhaps fulfilling the condition of the virus.
The events of Cascade suggests the virus was outlining the simultaneous destruction of both universes. The fact that each universe contains the other in its loop may refer to the interconnection of both universes - and how that interconnection led to their destruction.
Additional significance Edit
Due to the fact that the virus killed all of the troll's lusii, the events of the alpha timeline transpired in a manner which included the death of Gamzee's lusus. In an alternate timeline, where the virus was not run, Gamzee went murderous much earlier, before the completion of Sgrub, and proceeded to kill all of the other trolls, with the exception of Aradiabot, who went back in time to prevent the timeline. This Aradia had a hand in the virus being run and/or the subsequent explosion, as this is where the timelines split, but it is currently unknown in what manner she accomplished this, as she was not seen at that point in Hivebent.