The virus was actually a product composed by Jetan Remsen in his continuing plan to overthrow the Syndicate. It's proposed use was to destroy a Borg drone's implants and further augments freeing said drone from the Borg's Collective Consciousness. There was a genetic encoded suggestion for those infected to obey Jetan and not harm him as illustrated - during the Broken City episode when he coaxed Captain Rick Barlow to attack him. However, Jetan did not realize that destroying the mechanical side of a Borg drone kills the biological. The virus, however, did succeed in one case - freeing Colin Byrne. The Progenitor Virus had a side effect that devolution in a infected being's RNA. This caused those not augmented by the Borg to de-evolve into baser ancestral species, as when it infected the Voth turning them into their evolutionary cousins - prehistoric dinosaurs.
Jetan had developed an antidote, but its treatment was not ideal and was used as an inoculant before exposure. Gabriel "Matrix" Logan was able to re-engineer the antidote using the stem cells after the birth of Amelia Moore's and Rick Barlow's daughter, using a portion of the umbilical cord as a sample to make the antidote biologically stable enough to be rendered as a vapor, capable of being delivered by air.