Spontaneous Human Combustion (人体発火現象, Jintai Hakka Genshō) is a worldwide phenomenon that appeared shortly after The Great Disaster which causes humans to instantaneously burst into flames. In some cases, humans can adapt to the flames and awaken the ability to manipulate flames or ignite themselves or objects. The origins and selectivity of the phenomenon are unknown. The Special Fire Force have made it their mission to unravel the secrets behind it.
Typically, when a human falls victim to SHC, they transform into creatures known as Infernals. While in this state, the victim is able to create fire at will and is effectively immortal unless their core, located in the chest, is destroyed. The change also leaves them in constant pain, which forces the majority of cases to lose their self-awareness and sanity - turning them into rampaging creatures attacking indiscriminately or driven by simplistic desires from their human selves unless stopped by force. However, some victims are able to remain in control of themselves to a certain degree or, in cases with a notably strong will, remain in complete control of themselves and use their new body freely. As if to naturally combat this condition, humans have also begun to adapt to SHC. People born with the ability to manipulate flames are known as the Second Generation of pyrokinetics while those with the ability to create and control their own fire are known as the Third Generation. As both groups are partly resistant to being hurt by fire, people in Tokyo often end up becoming Fire Soldiers in the Special Fire Force; though not always.
One of the key mysteries about Spontaneous Combustion is what caused to appear and how it spreads. While in some cases it is totally random and sudden, it is also possible to deliberately trigger it using a certain type of insect. Once stung, the victim either transforms into an Infernal or awakens their pyrokinetic abilities.
It is confirmed in the series that it is the servants of the Evangelist who are at least partially the cause. Spontaneous Human Combustion occurs due to the encroachment of a persons doppelganger, their counterpart that exists in the world of Adolla, into the physical world. Little else is known about the phenomenon beyond this, such as the difference between the creation of an Infernal to the granting of Pyrokinetic abilities. The ultimate goal of the Evangelist is to utilize the Adolla Burststo force Earth and Adolla to completely overlap, and the Great Cataclysm was an incomplete and failed attempt at this. The world previous to the Cataclysm was so different in nature to that of the world of Fire Force (in that it resembled the real world, visually) that the effects the phenomenon that Spontaneous Human Combustion arises from seem to be much more mysterious than previously assumed.