It was just a normal night at Gray Mann co. But this place hides many secrets underneath this building.

An underground laboratory was built to create technology to destroy the RED team. They were testing on a new biological weapon that would kill the whole RED team, even the whole world.

Many cowards didn't want to volunteer, but there were some brave enough, and so they did.
They were locked in some glass rooms. Some kind of a virus was then put inside of them. They were getting sick. They kept vomiting and had strong headaches until they fell unconscious. Then, their skin became dark gray-ish, their eyes turned red and they couldn't talk, just making sounds. They became zombies.
They made a wrong decision choosing brave and strong men. The zombies shattered the glass and started biting everyone in the laboratory and infecting them. They somehow managed to get to the surface and then the outbreak started.