8. Green Flu – Left 4 Dead
Referred to simply as “The Infection” by the characters in the game, the Green Flu caused most afflicted humans to turn into rabid, zombie-like creatures. A few individuals afflicted by a mutated form of the disease turned into even weirder, superhuman zombies.
However, a few people are immune to the symptoms of the virus despite being carriers. The survivors of the game, Left 4 Dead, and its sequel, are suspected to be carriers who turned those they came into contact with into mindless zombies.
Research to combat the viral outbreak can be seen at various points in the game, showing humanity’s (losing) fight against the disease.