Forced Evolutionary Virus

The FEV or Forced Evolutionary Virus was created in Vault 87.The FEV virus is the reason Super Mutants exist.The fist test subjects of the virus were some of Vault 87's residence.Very quickly the FEV spread across the entire Vault,resulting in turning all of it's residence(including the Overseer) into Super Mutants.After this the Super Mutants found their way out of the Vault and quickly spread across the Capital Wasteland.