First you said 15% of enemies in TLOU are infected, and now you're just saying 'the majority of the game does not focus on the infected' so you could be referring to a 49% percentage.
Make up your mind.
Regardless of the exact percentage, which I estimate is pretty close to 50/50, the dystopian apocalypse setting is due to the infection outbreak and led to the collapse of society, countries, and probably the drastic lowering of the planet's human population... (Hence the title 'The Last of Us') and the humans left are as much at war with eachother for resources and survival as they are with the infected.
That's why TLOU is a zombie apocalypse game.