Oh no, a random virus has broken out of the lab and now it's spreading everywhere! The virus is too strong so they infected EVERYONE! You are now the last man (or woman) standing, and you will be battle an endless wave of zombies! Basically 1vs7billion+.
Be careful: The zombies are not what you think, it's attracted to noises, your blood, even the slightest bit of movement might trigger them. And guess what, it doesn't burn in sunlight, and also could spawn IN THE MIDDLE OF THE DAY. Oh yeah, have I mention about bloodmoon? Bloodmoon will spawn 3x zombies in the night, so good luck!
Basically Forge Labs' modpack, but I removed some of the things that will lead to your game being buggy.
Watch this man's video on how to "mutate" the virus!
https://www.youtube.com/watch?v=yRXWfmcDSwI&feature=emb_title
WARNING: This mod needs Optifine in order to work, if you don't have them, the game will just crash on you.
Also the modpack contains The Lost Cities mod, so have fun with that.
THE ORIGINAL MODPACK: https://www.curseforge.com/minecraft/modpacks/zombie-apocalypse-fast-zombies-by-forge-labs