I recently discovered Gbombs 5 on workshop. There is an entity in the "Chemical" section. It's a T-Virus tube from Resident Evil. People in close proximity of the tube will get infected only if the tube shatters. People who gets in close contact with an infected can get infected as well. After an amount of time, they (the infected) turn into regular HL2 zombies with MASSIVE HP and sped up movement speed.
The result is either the infection spreads so effectively that it takes over your server, or you stop the infection, one bullet at a time. Or just fucking nuke your game.
I recommend DarkRP server devs to implement the T-Virus tube as a buyable item. Make it cost high so players don't abuse it, of course. It'd be fun to watch server mayors pushing lockdowns on players if an outbreak were to happen. ya know, so players quarantine themselves away to not infect each other.