The basic idea of the story is great - two sides of humanity fighting against each other, the Pure and the Unpure. Now, clearly the 'unpure' disease needs to be very infectious and infect a large amount of the population - perhaps take on a 'I am Legend' type story here, where humans (YNC) attempted to create a permanent cure for a disease e.g. malaria, and this mutated into an airborne disease which spread across the planet infecting the population, with only around a 5% immunity rate. Perhaps a story like below:
The year is 2033, and despite major technological and biochemical advancments in recent years, there are still over 2 million deaths each year caused by malaria. It is now the single biggest killer on the planet, with over half of the Earth's population now at risk. YNC Services, a major biological engineering company owned by the Government, began working on a cure.
Initial tests were successful - the scientists altered the behaviour of mosquitoes and modified them to become stronger and superior to the current mosquitoes, while killing any Plasmodium inside of them, removing them as a vector for malaria. These mosquitoes carried their own infection, which would mutate the bitten mosquitoes into one of the super-mosquitoes - they attacked their own species, meaning it would only be a matter of time before the malaria carrying mosquito became extinct.
After extensive testing, the G8 decided to release the mosquito into the wild. And it was successful. Only a few years later, original mosquitoes were almost completely extinct. The super-mosquitoes, now without other mosquitoes to feed on, were forced to change to a different food source. Us.
A single bite from a super-mosquito was all it took to transfer the virus, mutating it's host into a violent, blood thirsty being, where the virus could then be transferred once again to another host.
Initially, the Government attempted to cover up the massive blunder by forging stories. But it wasn't long before the infection spread too far to contain. A worldwide pandemic began as the mosquito virus spread throughout the population. Governments began to quarantine towns, cities, and finally, entire countries. But it was not enough. Each of the countries' leaders set up safe zones, underground bunkers where those immune and who had not been infected remained in a safe environment, safe from the frenzied, blood crazed world above.
I was born in City 13, Bunker 57. All I know of the world above is what I have learnt from public information stations. We are safe here, but for how long? It is only a matter of time before the Infected find us. My name is...... I was born here, and will hopefully die here too.
Hope you like the story, sorry it was a little long, I got slightly carried away
[b]FPSC MIGRATION: http://forum.thegamecreators.com/?m=forum_view&t=142497&b=21