Diseases and viruses

Diseases and viruses are various conditions, although not mutations, throughout the Fallout series.

History
Diseases and viruses have played a pivotal role in shaping the world as it was before and after the Great War. Pre-War, various diseases and viruses were often weaponized for use in biological warfare. Old diseases would be modified into entirely new strains deadlier than before, and new diseases would be created in labs to gain the edge in countering offenses on frontlines. Both the United States and China would propagate the use of these weapons of mass destruction during the Resource Wars, amassing massive arsenals of hundreds of pathogens and toxins, until ultimately escalating and taking the final step into nuclear warfare in 2077.

Forced Evolutionary Virus (FEV)
From the biological arms race, the Forced Evolutionary Virus was created after being spun off from the Pan-Immunity Virion Project with the original intention to create a cure for New Plague, another biological weapon. The Forced Evolutionary Virus was a powerful mutagen capable of mutating any organism is came in contact with. FEV would forever change the environment of Earth as it would be released from many of the labs that contained in the Great War and would most notably be used by The Master to en masse create super mutants in New California from 2103 to 2162, finally being discovered and stopped by the Vault Dweller.

Continuing research into FEV, the west coast Enclave would later refine FEV into a deadly viral agent called FEV Curling-13, under the supervision of Dr. Charles Curling. This version of FEV was completely lethal and would kill all uninoculated humans exposed to the agent. The Enclave would be stopped by the Chosen One from releasing Curling-13 into the atmosphere in 2242. In the process of ceasing samples of FEV, the Enclave would inadvertently create another generation of super mutants comprised entirely of kidnapped people used to excavate Mariposa Military Base.

In 2277, John Henry Eden would reform the Enclave anew. Continuing off of the goals of previous west coast Enclave and using the research on Curling-13, Eden would further modify the FEV strain into a deadly water-soluble virus with plans to use Project Purity to release the modified FEV to the unsuspecting populace of the Capital Wasteland. President Eden would ask the Lone Wanderer to infect Project Purity with the FEV, but it's unknown what the Lone Wanderer chose.

Before the west coast Enclave would create Curling-13, the Appalachian Enclave would use samples of FEV taken from the nearby West Tek research center (experimenting on the town of Huntersville to create early super mutants) and expose pygmy bats to the mutagen turning them into Scorchbeasts as well as creating the Scorched Plague. President Thomas Eckhart would release the scorchbeasts into Appalachia to gain access to the remaining nuclear arsenal of the United States. The release of the scorchbeasts would, in turn, propagate the Scorched Plague and nearly wipe out all life in Appalachia, but the scorchbeasts and scorched plague would be stopped by the residents of Vault 76.