It was supposedly an artificial intelligence in the form of a computer worm that could infect any electronic system, modify itself, "reproduce" and spread nearly uncontrollably. Basically an electronic life-form, one which took over the infected systems/ships and tried to eliminate organic life-forms.
From the Traveller Wiki:
The Virus is a living, thinking being that moves through the medium of electronic information and computing systems the way a fish moves through water.
The Virus is not a biological organism. Imperial scientists have taken to calling the Virus an 'Infomorphic Vampire' and consider it a sentient being, if not life, by the traditional definition.
Basically, it was the authors' toy to completely deconstruct the setting to allow them to change whatever they wanted - how tech works, ship designs, social systems, interstellar polities - nearly everything.
Yep - it was Traveller's counterpart to The Spellplague/Great Rain of Fire.
I can't think of any way to use this without "blowing everything up".
Unless you put it into a historical "this started once, millennia ago, but it was stopped quickly and we think
it was completely eradicated" - and maybe have the PCs trying to stop someone who is looking for a rumored surviving quarantined sample of the program.
Catapultam habeo. Nisi pecuniam omnem mihi dabis, ad caput tuum saxum immane mittam.
"I have a catapult. Give me your money or I will hurl a large rock at your head".
"Buffy, Blade... its up to you now." George Takei
The only time a Vampire should sparkle is right before they explode