I work from the assumption that someone somewhere in the chain of nuclear reaction targeted southeast Asia for some reason, or there was an accident: maybe a US bomber went down on it's way to China, or nuclear elecromagnic craziness made a missile crash off course. Also if the US was invading Canada they were probably doing cold war shit all over for years beforehand, there could have been western nukes hidden in the area already. Maybe one was an old GLADIO backpack nuke set off as part of some harebrained forgotten strategy of tension, or by a lone actor for reasons lost to history. For whatever reason, let's say that Thailand got one or two glancing hits, nothing too direct. Most people survive the fallout, but must deal with a surprise almost as nasty: a secret American black ops base in the deep jungle that was chock full of FEV.

Maybe the commander there thought the worst of the nuclear fire was yet to come, and ordered it released in the belief that they had only minutes to try and give biological life in the area a chance to survive. Maybe they knew that this region would be mostly unscathed, and in a fit of rage decided to punish these people for the good fortune not shared by their Imperial homeland. Either way, every vertibird was scrambled with a canister of FEV, and they spread it over the jungle in all directions. Dropped the empty canisters in the river for good measure. So despite surviving the nuclear exchange, Thailand got a big fucking dose of FEV.

The virus rampaged through the ecosystem, from water to microbes, insects and birds, herbivores, predators, and human beings. It would be the primary hazard of the game in most places, akin to rads in other games. The vast majority of afflicted beings simply died riddled with massive tumors or hemorrhaging from the rapid formation of new and useless organs. Of the survivors, most had children that were born with mutations ranging from minor to severe. But the extreme nature of the virus meant that each generation after it's original survivors stabilized faster than the last. During the 200 years much of the rest of the world was crawling around in irradiated ruins hunting bug meat and sleeping wrapped in the flags of a dead world, an entirely new kind of ecosystem was beginning to shape itself in Thailand. Gene pools stabilized, new food webs emerged, new microbes and mychorrizae developed new relationships with new flora and fauna.

The descendents of the first FEV victims -plant, animal and human- had come to terms with the new environment. Some plants became violently carnivorous to compensate for the lack of sunlight from worldwide nuclear winter. Some people developed enhanced senses for safety from new and lethal predators, or longer legs to walk through the waters, or extra arms crowned with natural tools and weapons. Some animals became unfathomably deadly, while others were elevated to undeniable sentience. The elephants gained psychic abilities, and in places began working together with the humans voluntarily, each species making up for the shortcomings of the other. Moat humans do the work that requires physical finesse (such as crafting) while most elephants do the work that requires either inhuman strength or unimaginable psychic empathy (like diplomacy). Together they build a new thriving society against the many threats of the new environment.

The black ops base was also exposed of course, but it's been shrouded in rumor and mystery for 200 years, deep in the densest mutated jungle. Nobody outside is sure if it ever really existed at all. The soldiers there have spent the centuries locked underground devolving into a militarized quasi-society of cannibalistic combat monsters, supplementing their FEV with steroids and various poisons from the in-house lab. Some mutated while in their power armor, and have become horrifying amalgimations fused forever into their hulls. Death is the only purpose they can remember, and they are preparing for the day they unseal the bunker and bring it to the world.