Virus was the "final solution" superweapon developed under Lucan's direction and intended to end the Second Civil War (1116-1130 Imperial), also referred to as the Final War. End the war it did, but not in the way Lucan may have intended.
For most areas of the former Third Imperium, exhausted as they were from the struggles of the Final War, the crash was hard and fast, especially near the areas the weapon was released. Most areas were reduced to pre-spaceflight technological levels. Many worlds which could not support their populations without technological aid died out within the first few months of the outbreak of Virus.
Only the former Domain of Deneb, through its relatively isolated location and strict quarantine of its borders, remained relatively intact. About 1220 Virus had died out enough that space travel became feasible again. The Reformation Coalition and other pocket empires began to expand slowly into space again, destroying any Virus-controlled ships or systems they encountered.
In further research of the chips, Imperial researchers noticed some of the chips had developed into predators of others. They had the ability to use radio signals that interrupted a chip's own control actions and forced the other chips to overwrite their programs with those of the predator. Virus was a further development of this natural ability.
Virus has the ability to transmit itself across any radio frequency and force computers receiving the transmission to hard-code the Virus onto their chips. Once a foreign system had been successfully infiltrated, Virus was designed to cause as much destruction as possible before destroying itself.
Most of the initial infections of Virus were made through the transponders of starships. Because starships are the fastest means of communication in the Traveller universe, news of the existence of Virus usually arrived at the same time the Virus itself did.
After its release Virus quickly mutated into strains with various interpretations of its basic instructions, many of which no longer had the instinct to destroy itself. "Vampire Fleets" of infected warships roamed the space where the Third Imperium had formerly been located, fighting other Virus ships for more systems to infect. The disintigration of interstellar society was the result.
Virus is intelligent, and because of its original programming is usually unremittingly hostile towards all other life. Some strains of Virus were able to develop a more peaceful attitude towards other sentient life, but very few strains could be considered "sane".
By the 1220s most of the more virulent strains of Virus had destroyed themselves or been unable to maintain their systems any longer, and it became relatively safe to travel again.
Many fans also scoffed at Virus' ability to write itself over any computer system as unrealistic.