Sers Leute hab mal bißchen bei google gesucht und tatsächlich was gefunden wie man in der Demo die Infected spielen kann. Hier ne kurze Anleitung:

This plugin works with Single Player, Dedicated Servers and Local Servers. Here's what you gotta do:

1. Extract the ZIP to \Steam\steamapps\common\left 4 dead 2 demo\left4dead2 InfectedPlugin.zip
2. Start up Left 4 Dead 2 Demo
3. Get a game going in Single Player or Online.
4. Type the following into console:
plugin_cvar sb_all_bot_team 1
plugin_allow_remote_commands 1
5. To become Infected, type the following into console:
set_infected 1
6. To change back to survivor, type the following:
set_infected 0
7. You can force a client in your server to be infected/survivor by using:
plugin_set_infected <their userid> <0 = make survivor, 1 = make infected>
8. Don't want your clients to be able to make themselves infected and manually spawn? Then use the following command:
plugin_allow_remote_commands 0
9. If you want your clients to spawn themselves by using infected_spawn, and to disable auto-respawning, use this command:
plugin_infected_autorespawn 0

Aber nicht vorher vergessen die console so einzustellen das man sie ingame öffnen kann. wie das geht weiß bestimmt jeder (hoffe ich)

Achja nochwas das ist dann kein Versus!!! man spielt dann gegen die Bots!! Aber macht Spaß mal die neuen Infected zu Spielen!!!

Nix Cheater!!!