Research logs at Arktos Pharma seem to suggest that the Scorched Plague was created by them before the war. Whether it was designed as a bioweapon or accidentally created as a byproduct of other experiments is currently unknown, but this research was later found and further developed by the Enclave under Thomas Eckhart.
The plague is responsible for the destruction in one way or another of almost every human faction that had existed in Appalachia, from the Responders and Free States to the Brotherhood and raiders; even the Enclave at the Whitespring Bunker wiped themselves out in a civil war over its use.
The fungus has some inexplicable connection to the hyper-radioactive ore known as ultracite; the characteristic green crystals that penetrate from the flesh of the Scorched are ultracite crystals, formed from the fungus itself as it ages.
The Scorched Plague is a highly contagious mutagenic fungal pathogen that has infected almost all of West Virginia's human population, turning them into the scorched; beings similar to feral ghouls that retain enough presence of mind to wield weapons and wear armor to assault any uninfected humans unfortunate enough to cross their path. Creatures infected with the plague will not be hostile towards other infected creatures, regardless of species or race.
The plague is also responsible for mutating the local pygmy bat population into scorchbeasts; giant bat creatures who, having overpopulated the underground caverns they lived in, spread the plague to the surface. It seems to have become symbiotic with the scorchbeasts because these beasts retain their individuality while other infected creatures become part of a hive mind, connecting them to the beasts and giving the beasts the ability to control other infected beings to some extent.
The plague also infected other species of local wildlife but didn't mutate them to the same extent as scorchbeasts. A few species such as mole rats, wolves and feral ghouls are noted to have a particularly strong resistance to, if not complete immunity from, the plague.
At least in humans, the Scorched Plague continues to grow inside of the infected, causing ultracite growths to cut through their skin, eventually overtaking the host and killing them. This internal lacing with ultracite also makes plague victims vulnerable to ammunition made from depleted ultracite, due to the negative reaction that occurs when normal and depleted ultracite are brought together. This weakness was leveraged by the Responders' Fire Breathers division, who laced munitions with depleted ultracite crystals, and the Brotherhood of Steel who created laser weaponry with ultracite crystal focus arrays, dealing extra damage against infected creatures.
The plague starts its life cycle as a spore which then grows into a fungus. As parts of the fungus age it hardens into ultracite. This allows for easy tracking of infected creatures, as ultracite gives off a distinctive radiation signature. Ultracite then interacts with the fungus, strengthening it and the creatures it has infected. However, the fungus can't tell the difference between depleted ultracite and normal ultracite, which still has some properties of normal ultracite, but not the same nutrients that the fungus needs, which makes it weak and slows its life cycle. Scorchbeasts have been observed guarding ultracite deposits and consuming raw ultracite ore, suggesting they are smart enough to realize consuming ultracite strengthens the plague fungus inside them.
While the Responders, Free States and Brotherhood were all developing an inoculation/cure for the Scorched Plague, there are some creatures with a natural resistance to the disease.
The Scorched Plague appears only in Fallout 76.