Hopefully, Back 4 Blood’s story will continue building on the mystery of the virus in a way Left 4 Dead never did. Discovering the origins of viruses in zombie fiction is always an intriguing mystery. Like the Green Flu virus, this parasite causes aggression and triggers mutations in its host. While studying it, the parasite got loose, infecting scientists and kicking off a worldwide outbreak. In the reveal trailer, we learn of a parasite discovered frozen in Antarctica that may or may not be extraterrestrial in origin. For Back 4 Blood, Turtle Rock Studios has been much more open about its virus. Where it came from and how it infects the body was left up to the imagination. The origin of the virus, however, was left a mystery. The diseases caused aggression, mutations, and zombification, leading to plenty of intense encounters against normal and mutated enemies. Left 4 Dead and its Valve developed sequel, Left 4 Dead 2, featured plots that focused heavily on groups surviving an outbreak known as the Green Flu.
