The virus is once used to be called Redlight. It has infected Elizabeth Greene, and this is how to inspire the creation of a bio-weapon research program.
This leads to the birth of Gentek, and Alex Mercer work for that company. He has recreated the Redlight virus into Blacklight virus. This type of virus is 10x deadlier, but it's design is to rewrite living creatures upon infection.
Unfortunately, many scientists of Gentek are killed by Blackwatch because of a leak. As a result, Alex fled with a sample of the virus as "insurance" to Penn Station. However, Blackwatch corners him there; knowing this, Alex unleashes the Blacklight virus instead of surrendering. This results in his death and the birth of Zeus.