Go to versus mode fon left 4 dead press choose character and then infected then when the host starts the game you can be an infected NOTE:you can only be Boomers,smokers,hunters and tanks
Other note to play versus on splitscreen(Xbox 360 ONLY) press play with freinds make a match and change it to a lan game
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I believe the story behind Left 4 Dead is that a new strain of Rabies has spread probably through biting or it is airbourne. The 4 survivors are immune to this disease so that's why the can get attacked with out turning. You can also play as infected against the survivors but you cannot play as regular infected.
**EDIT BY "BLACKQUAZA"**
while no storyline answers how the regular infected become this way, it DOES explain how special infected are created. It seems that when you attack a Survivor, you have a small chance of giving them the disease. For instance, I vomited on a survivor, and escaped. Vomited on him again, and again escaped. I vomited on the same survivor a third time, and a little message said I gave the Infection to Player, and I got an achievement. Unfortunately he died 20 seconds later so I don't yet know what it does. But my point is, when attacked they CAN get infected, but it's a small chance, and the protagonists got lucky. Really lucky.
The user is partially right, but the infection started by an infection from insects. As you can see in l4d one there is a green paste that happens to be insect slime. After the bugs wore off, infected changed survivors into infected. The lucky survivors are immune and have a small chance, like the last user said. There are many stories told in different ways, about the starting of the infection. (Ntutusfrs version on steam group.)
what console were you using to get that achievement?
I was using a computer when it happened. I was on the final level of Blood Harvest when it happened, in case you were wondering that too.
i heard that the infection started when scientists were messing around with a new chemical, and testing it on rats. one of the rats escaped, and infected a person. that person infected another person, then they infected 2 more...until only the people that were immune to it were left.
In left 4 dead, the Zombies aren't actually "living dead."
A strange disease has turned some people into common infected (a.k.a. the zombies that die easily and come in huge packs.)
others were mutated by the disease, such as
.tanks ( large, strong, ripped zombies that look like the thing)
.chargers( about the size of tanks, gray, one huge arm, one tiny arm)
.jockeys ( small, fast zombies that jump on your back and ride you as they claw you)
.spitters ( a female zombie with a bikini and an unnaturally huge mouth, spits acid)
.smokers ( a gross zombie with a freakishly long tounge, constricts you with it)
.witches ( crying zombies with long claws, if you disturb them all hell breaks loose.)
.hunters ( zombies wearing black hoods that jump onto you & rip you to shreds)
.boomers ( big, fat zombies which vomit on you and attract large swarms of zombies)
these zombies don't die as easily...
well, boomers do, but if you shoot them from very close vomit shoots out onto you
if you shoot them from far away, haha, it rains guts :)
You can only play a zombie in online versus mode. Competing teams will automatically switch between playing survivors and zombies between rounds.