The Blue Pandemic was a deadly sentient virus that was created by the tyrant Gorgon with a piece of his consciousness imprinted on it as a last resort to destroy any and all opposotion against his and Oceanus' co-ruled Legion Empire. Gorgon successfully spread the virus through Rapture's ventilation system an hour before a group of Splicers detonated a nuclear bomb killing him. However the virus failed in its duty to destroy the Empire's enemies as the virus killed everything in it path - Big Sisters, Big Daddies, Splicers, Tritoms, Legion - it made no difference who or what it killed.
Eventually a vaccine was created and though it didn't destroy the virus it did push the virus back leaving it concentrated in Blue Abyss where The Legion claimed as their new lair. Gorgon's mind resurfaced in the virus forming it together as a blue cloud and reclaimed control of The Legion and The Blue Abyss. A few years later the the dethroned tyrannical Triton King fled to The Blue Abyss were he found Gorgon in his phantom-like virus body. Gorgon made a deal with the Triton King that if he let Gorgon take complete control of his body he would make the greedy Triton all-powerful and he accepted. The possesion process was successful but the Triton took back control of his body and The Blue Pandemic dissolved into his bloodstream mutating his body, killing Gorgon and destroying the virus in the process.