In the 22nd century, the major world powers are at each other's throats, fighting over dwindling resources and territories. One of the sides develops a biological weapon, let's say a virus, to be released among enemy soldiers/settlements. But as these things go, a flaw in the design causes the bioweapon to become much more dangerous.
What I want is for this biological agent to kill billions, leading to a mass extinction event among humans. Furthermore, I also want the following effects:
- The majority of people that come in contact with it die - on the scale of billions
- A smaller group of people are immune to it - maybe due to something in their blood or organs or whatever
- Yet another group of people are affected by the virus, but don't die. Instead, they are physically affected - disfigurement, constant agony and shortened lifespan. Maybe they discover that a blood transfusion from the immune survivors can somewhat counteract the effects, which would lead to the main conflict
How could the above potentially work?