Based on the original FEV-II recovered by the Enclave at Mariposa Military Base's ruins, FEV Curling-13 is a viral agent tailored to reach maximum levels of lethality with minimal exposure levels required to achieve the result. It was being developed under the direction of Lt. Col. Dr. Charles Curling of the Chemical Corps since 2236, to provide the means for the final phase of the Enclave's Project.
Unlike FEV-II, which required a severe overdose and low radiation count in the subject to work correctly, Curling-13 killed indiscriminately. Just a .0001% aerosol solution exposure was enough to kill Arroyo villagers in under one hour, as massive hemorrhaging and inflammations start, leading to a complete failure of the victim's organs within sixty minutes. No test subject survived the tests on the Oil Rig.
Significant radiation-induced mutation may be beneficial if exposed to the virus, however - pure humans would suffer a much worse fate. Pure Vault 13 dwellers exposed to the virus expected subdermal hemorrhaging and convulsions within sixty minutes of exposure, but their agony lasted much longer - 14.5 hours on average.
The Enclave's manufacturing capacity was tremendous - within six weeks they could produce 250,000 gallons of the virus and unleash it on the entire post-apocalyptic world just by spraying it into the jet streams converging at the Oil Rig via Vertibird. The winds would then carry the virus worldwide, achieving global saturation in two short weeks. Only humans and human-based mutants would be eradicated, however; FEV was engineered to bond only with human glycoproteins, leaving animal and plant life alive and quite kicking.