A Game of Survival in an Empty World
From the introduction:
You sometimes wish you knew what white-coated expert tucked away in some biological warfare lab had come up with the idea of the superplague. But it doesn't really matter much; he's probably dead, like the rest. Once the superplague broke out of the Level Four facility, it spread too fast for anyone to ask for a government investigation.
No one will be asking for one now; there's no government anymore, either.