These commands used to be available in L4D1 through the console, but was removed for reasons I'm not sure of, but are still there. The only way to be able to modify these CVARs, is through the admin menu and system mods.

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=574484797

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=214630948

When you normally play versus and there is no human players on the infected or human team, the server shuts down and the game ends. To prevent being kicked, type this command into the console:
sb_all_bot_game 1

It's also best to make sure you have enough switches just in case something happens or you switch back to the survivor team to do something such as activate a finale or to make them progress a little to a better location. Use this command:
vs_max_team_switches 99

This is when you're gonna need to use the admin system to swap between the game modes. Type this command into chat:
!cvar mp_gamemode versus
What should normally happen, is that the game mode is now set to versus.

All you need to do next, is use the jointeam 3 command into the console to switch to the infected. Or use the built in keybinds to switch.

(I suggest moving the survivor team forwards out of the saferoom before you switch teams so that you can spawn in as a special infected. You can also use the console command warp_all_survivors_here to teleport them to a location usually near your cursor.)

Once you have switched to the infected team you need to switch the gamemode back to campaign OR survival if you're on a survival supported level on a campaign. Use these commands in chat:
!cvar mp_gamemode coop
!cvar mp_gamemode realism
!cvar mp_gamemode survival

Now you'll quickly notice once you're gunned down by the survivors that you aren't respawning. That's where the issue of campaign comes in. Normally players are not put into a spawn queue like the rest of the infected. There's two ways to fix this issue, but it's a bit tedious. The quickest way, is to just use the built in admin system menu by pressing i, to spawn in infected.

Another way you can do this, is using the z_spawn commands in the console, such as:
z_spawn hunter
z_spawn tank
z_spawn boomer
z_spawn smoker

So far, I've only found 3 noticeable things while playing as the infected in campaign.

When you play versus, normally the thirdpersonshoulder command does not work, forcing you to play in firstperson to prevent any advantages thirdperson can give such as looking over corners. However, since you are playing in the campaign, this command is re-enabled, allowing you to use thirdpersonshoulder as the infected.
You are most likely the aware the hunter can pounce away in a yelp when you shoot at him before he tries to attack you in the campaign. This ability is normally disabled in versus, but can actually be used in the campaign by the players. When you left click, you will pounce in a yelp, and this pounce can actually be used like a regular pounce when it comes to wall jumping, but when you try to pounce a survivor, you will actually push them! I found out this bugged push a few years back before, but I didn't know how to recreate it because all it did was create one of those hit markers in the left of the screen:
[Player name] pushed Louis x1

When you play against the tank in the campaign, you will notice that whenever he downs a survivor, he does a short angry animation, this does not happen in versus. The tank also does a victory dance once all the survivors are downed in the campaign, this also does not happen in versus, but when playing as the Tank with these methods, you will actually perform these short animations whenever you down someone or trigger a game over.

I also figured out that with the admin menu or the console, you can disable the director, which not only stops things from naturally spawning allowing you to make your own campaign-ish game if you have ai improvement mods and you're on the spectator team, you can also spawn a way larger amount of special infected than you usually are allowed to.

Other mods to check out!

Been a good while since I first made this guide and there is now easier ways to do this and even have it work while playing with friends or with bots (with the mods required to let the bots move on their own and not end the game on player death that is.) so check these out!

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3381291531
If a special infected spawns when you are currently dead you will spawn in and take control of that infected, allowing you to mess with your friends who left you for dead.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3062675250
manually swap teams for endless enjoyment of riding infected as a jockey on the survivor team.