How did the infection start in left 4 dead?

Answer 1

The Zombies you see in Left 4 Dead 1-2 are infected with a rabes-like disease that makes them very destructive and have a desire to infect everything that is healthy. The zombies have already found out that you (The Survivors) have an immunity to the disease that they carry, so they decide to kill you instead.

This disease apparently randomly came into action on the Earth and since then has infected over half of the world's population.

Answer 2

my gamertag for xbox live is Xxvictor619xX add me i play skate 3 and mw2 the infection started when a deadly chemical was released and infected most of the worlds population. the people did not know they were infected until they turned cold and died. the special infected did not start of like that. they were normal infected but evolved. beive it or not the boomer was actually skinny.the survivers cannot catch the infection as they are immune so the zombies kill em. But if you are infected your brain will go out of control and you do not know what is going on. the infection is a bit like rabies because you will go mad and infect anything you see!