How do the zombies in left 4 dead get infected

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.