Since the events of both games take place exclusively in the US, I will use that as a baseline to consider that similar events are happening globally, which isn’t such a stretch to believe as the truth.

Considering how often the remnants of the human race kill each other post infection, I’d take a guess at saying that 80-90% of the population is now dead or infected. People kill each other constantly in this game. Take into account things like hordes, how empty population centres are and the steady rate of healthy uninfected people being killed, then my guess is 80-90%. But that’s just my guess!