Like its predecessor, Left 4 Dead 2 is set shortly after a worldwide pandemic of an infectious disease that rapidly transforms humans into zombie-like creatures and mutated forms that demonstrate extreme aggression towards non-infected (much like the ones in 28 Days Later). Few humans are immune to the disease, still carrying the infection but showing no symptoms.
The Source engine powering Left 4 Dead 2 isn't quite as demanding as a few of the engines on test today. We are therefore able to run this game well at 720p, even with onboard Intel HD graphics.