Killing Floor/WMG

It's Green Flu

The Virus has spread to both sides of the Atlantic. The U.S has to deal with Hunters, Boomers, Smokers, Chargers ETC and the U.K has to deal with Sirens, Scrakes, Bloats, and Fleshpounds. That's because the Green Flu has evolved in different ways in different regions. Oh yeah, and Horzine is responsible for releasing it into the U.S.

Green Flu can turn people into cyborgs?
- If it can turn people into things that can puke out tons of zombie attracting slime every 30 seconds, yeah, it probably could.
- It completely goes against defined canon, but hey, why not. Plus, Horzine could have been working on Green Flu, the game says they're good at multitasking

The Trader is the sister of The Merchant

They have the same supplier, and the reason the Killing Floor zombies never attack her is because she has a plagas inside her, and out of professional courtesy the different zombie types leave each other alone and just focus on humans.

The Patriarch is a client of the Trader

She keeps him in bullets, rockets, and napalm - which lets her find out what he's up to. That's why she's always ready to sell weaponry in the locations of impending battles, and why the Patriarch and his minions actually leave her alone.

One whitelisted map goes along with this: the most recent version has four normally-usable Trader locations, with a hidden fifth one that the Patriarch always spawns from during the final wave.
This troper was playing West London, and as it turns out, the Patriach spawned in the Trader room my group had just been in. Possible fuel for the fire?

The Trader is The Patriarch's former wife.

After surviving a siren trying to kill her when she tried to divorce her husband, she barricaded herself in a room and dug a series of tunnels around London, and gathered weapons while everything went to hell above. Disgusted at what her former husband was doing, she started to sell weapons to the KF team to make sure that The Patriarch dies.

The reason the specimens don't attack her is because she, in the back of their minds, is still their mum, similar to why the specimens didn't kill The Patriarch.

Actually, it's stated that The Patriarch's former wife was killed. She tried to leave him and he used a Siren (which was cloned from her DNA) to kill her.

There will be a crossover with Left 4 Dead

Seeing as we have a Pyro character introduced in the holiday event, as well as the Portal crossover level in the Portal 2 Steam ARG, this is possible.

They've already ported the Killing Floor models into source and swapped out the Left 4 Dead 2 models with the two police officers. Given the similar settings, it makes a fair amount of sense that we'd have Ellis, Zoey, Francis, Coach, Louis, Rochelle, Bill and Nick standing shoulder to shoulder with some British Army troops.
- Actually happened (sort of). One of the 2011 Steam Summer Sale images had Francis and Rochelle teaming up with Mr. Foster and the Steam mascots to egg/toilet paper the neighboring Totally Not Zombie Camp.

Kevin Clamely/The Patriarch has PSL for The Stalker.

Both times the Zed have been reskinned, she's been reskinned as his counterpart (Santa/Mrs. Claus and Ringleader/Assistant, respectively). And in the Christmas Event group shot, she was sitting provocatively on his lap. Ew. But then again, he's a Nightmare Fetishist, isn't he?

To certain members of the non-Zed playerbase, the Stalker is somewhat considered hot (a couple of back. This is probably because the only other two women are ugly (the Siren) or are in the process of selling you guns with innuendo-laced language. And if *you* were the Patriarch, and you had to choose between the Siren and Stalker, which would you choose?

The Trader is a female Drebin from Metal Gear Solid 4: Guns of the Patriots.

Randomly appears in convenient warzones? Doesn't really take sides but still helps the heroes mostly? Has millions of guns for sale?

Nanomachines cause mutants. The Patriarch is a cyborg controlled by GW. Every player character is unknowingly a clone of Solid Snake. It all makes a twisted form of sense.

The Medical syringe is full of Bacta

It's in a small canister, has miraculous healing potential, what else could it be?

Ragnite?
I've never seen red bacta before.