Officially the SHIVA virus is identified in 2054 with its point of origin being somewhere between India and Pakistan, where it was believed to be a biological weapon and resulted in a nuclear exchange betwen the two countries.
A strain of the virus known as SHIVA II spread through the remainder of Asia as a result of this nuclear exchange and set off further nuclear incidents.
In 2056, approximately two years after its identification, a variant of the SHIVA virus reaches the Hoover Dam region where a great deal of the population die, however it has not been ascertained whether the SHIVA virus or protracted nuclear exchange was the root cause of this.
Overall the SHIVA virus appears as seven distinct forms, with naming conventions of SHIVA I through SHIVA VII, with the seventh version being mostly non-lethal.The virus is named after Shiva, the hindu God of death.
Speculation
The SHIVA virus, or at least SHIVA I, is hinted to be an airborne virus with a quick infection exposure time, and therefore has qualities much like a modern chemical weapon.
SHIVA VII acts as a human endogenous retrovirus, which in laymen's terms means that it has associated or attached part of it's genetic structure into the human genome, technically everyone has these and will have them since they are reproductively inheritable.
Human endogenous retroviruses can be removed from the human genome, however this will not effect the genome of currently existing humans. So in effect if one was to do this they would have to 'genetically manufacture' an entire replacement generation of humans that are SHIVA VII HERV free.
Realism Factor
The chance of a modern thermonuclear detonation assisting in the diffusion of an airborne virus is extremely unlikely, due primarily to viruses being highly vulnerable against mild radiation and extreme changes in heat and pressure.
Generally viruses do not change numerous times over a short period, and when viruses do change into other strains it usually results in impeding their capabilities to successfully infect further hosts to the same degree as the original virus.
When the above notions are applied to the Fallen Earth scenario it highly suggests that greater gears are in motion concerning the work of the SHIVA virus, and those are probably going to be key plot points within the game.