In order to play as an infected, you’ll first have to download the plugin from the l4dmods website, then follow the instructions below:
1. Extract the ZIP to \Steam\steamapps\common\left 4 dead 2 demo\left4dead2
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
<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
And now, the hands in the image you can check out in the upper left corner will be yours! Have fun!