Due to mysterious circumstances, and a suspicious bottle of pineapple lotion, a deadly virus was released onto the Normandy. The infected began to rot, their skin turning green and flaking off. As the virus progressed, they began to cough up blood, and, finally turn into mindless shells obsessed only with eating the flesh of their fellow crewmates.
Once the crew realized what was happening, at least one of them had obviously seen a zombie movie, and set up quarantine right away in the brig. Survivors barricaded themselves in the medbay.
Staff Commander Kaidan Alenko and Astor Cohen heroically and with varying degrees of sexiness drew the zombie horde away from the med bay before manfully making their way to the brig in order to gather samples from various infected crew members.
Cassondra Shepard searched for (and somehow rescued and didn't shoot) survivors, and ran samples from the brig to medical.