I'll tip my hat, you're an awesome TF2 sniper.. but you're talking FUD about L4D.
If the survivors run amok, separate and don't work as a team, they'll die horrendously. It's the same with the infected, only people haven't begun working as a team as the infected yet.
When playing as infected, you need to think like the survivors - work together, like they do, use corridors and entrances for cover like they do, stay in a relative group/formation, like they do. Attacking one at a time against a group will always fail, but the same is true for the survivors.