A soldier was stabbed with a needle during a mission. They started acting weird and after investigation, it turns out the needle had zombie cells in it. It eventually spread to the other soldiers and started an apocalypse

You, Gaz, Ghost, Soap, Roach and Price were standing on the roof of the base. Price was cursing under his breath, Ghost and Soap were shooting down the zombies below and Roach was planning how to get to a safer area that wasn’t a roof with no food or water.