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.

New Plague
One such new disease created by the United States would be limit 115 or as it would be commonly known as - New Plague virus. Created in the secret laboratories of Hoover Dam, New Plague would eventually enter the populace after Chinese spies stole samples. It was quickly released in an attempt to apprehend them but only ended up exposing the virus to a crowded area in Denver. which rapidly spread throughout the United States in the 2050s, eventually demanding a national quarantine in 2053. The epidemic became the driving force behind the Pan-Immunity Virion project that paved the way for the Forced Evolutionary Virus.

After the Great War, New Plague would be on the present factors (along with radiation, inbreeding, and FEV) in the mutations present in the Swampfolk, leading their severely deformed appearance as well as their aggressive nature to outsiders. The New Plague virus is likely still a dormant disease in many parts waiting to propagate throughout the population again or having evolved into an entirely new form as Robert MacCready's son Duncan shows symptoms similar to a New Plague infection.

Forced Evolutionary Virus
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.

The Enclave would utilize FEV multiple times throughout post-war history in an attempt to genocide all mutated human life on Earth or to either gain control of the remaining nuclear arsenal of the United States. Each attempt left the Enclave defeated or in ruin, either collapsing under their own machinations or being routed by the Chosen One and the Lone Wanderer. From their research and projects with FEV, the Enclave would create a second generation of super mutants and propagating Scorchbeasts and the Scorched Plague.

In the Commonwealth, The Institute would use FEV in producing another strain of super mutants. The experiments they conducted further advanced their research in realizing generation 3 synths after finding a pure specimen to draw genetic information from.