WMG / Crossed

The Crossed virus is ancient and predates human history.
The only basis I have for this is the covers, but on some of them it appears that ancient civilizations have succumb to the virus before(archaeologists finding cave drawings of the Crossed, the Mayans showing up Crossed, etc.) Perhaps the virus is far older than it appears, and is something that has been with humanity for as long as humanity's existed, occasionally causing outbreaks that result in mass deaths. And in relation to that....
  • Confirmed. Kieron Gillen's Homo Tortor arc mainly takes place 75,000 years before C-Day, so the virus is definitely ancient.
    • Actually jossed. Homo Tortor (at least the 75000 years ago part) is basically a fanfiction in-universe by a Crossed professor.

The Crossed virus caused many of the historical massacres and disappearances that cannot be explained.
From Roanoke Colony to the Mayans to any other event one can think of. Maybe they were caused by the Crossed virus "activating" somehow and killing off a large percentage of the human population.

The Crossed outbreak is God "correcting" Humanity back to its intended state
According to the Bible, God originally intended Humanity to have no conception of right or wrong and thus be completely innocent - the knowledge of good and evil was a corruption of Humanity that He did not want. The Crossed virus is His final solution - returning everyone to their natural, much more animal state, with only one extra impulse - to propagate the virus throughout the entire species as forcefully as possible. In a universe with God, everybody being Crossed is actually not bad - they all seem happy enough, and the constant death really doesn't matter since, everyone being innocent again, they'll all end up with God anyway. The X-faced nun from Wish You Were Here is actually his last Messiah, since she actually led a truly good life of total purity, she's worthy of Heaven without needing innocence, so she's the perfect candidate to infect with a variation of the virus that grants heightened intellect and awareness so as to lead the Crossed more effectively to claim the last survivors who have formed strong holdouts that leave them possibly safe from regular, chaotically organised and unstrategic Crossed groups.

Aoileann isn't Crossed at all
Wish You Were Here has already introduced the idea that some people might be immune to infection. If Aoileann is one of these, it would explain why she doesn't have the rash despite getting dangerously intimate with the Crossed. It also means that her experiences with Shaky and the Gamekeeper must have driven her to the kind of madness which can terrify even the Crossed. This also explains her companion in the home-made biohazard gear; whoever it is, it's Aoileann's ally, and it suits her for them to remain uninfected. Otherwise the regular Crossed would just tear them apart.
  • While we're on the topic, concerning her companion, it's most likely the Gamemaster under all that biohazard gear.
    • Jossed. It was Ashoke.

Aoileann is Crossed after all
She speaks in the same red text that all Crossed use and openly participates in the same debauchery as the other Crossed. The reason she doesn't have a rash is because of the "X" scar on her face. The cuts may have been deep enough to sever whatever organs (blood vessels, nerve endings, whatever) that cause the signature Crossed rash upon infection. So, Crossed without being crossed.

Aoileann is Crossed, but is different because she has epilepsy.
Epilepsy is a condition centered in the brain, the Crossed disease (probably) effects the brain. Coincidence? I think not.

The last panel of the last page of the last Crossed story
A wide shot of a vast plain, littered with bodies. Some half-eaten, some starved, some who died mid-coitus. The non-Crossed killed by the Crossed, and the Crossed finally dead from exhaustion, malnutrition, self-mutilation, or any number of things.

And the lone caption will be: THE ARISTOCRATS!

The Crossed are the result of a vengeful Jesus Christ revoking his forgiveness.
After Jesus died for humanity's sins, life continued much the same way it had. Billions of years later, most people are growing more skeptical of religion, or are twisting the Bible to suit an increasingly-secular society. Jesus learns of this, and gets pissed. Humanity are doubting his gift, perverting it and misinterpreting his word. So what does he do? He takes it back. The cross-mark on their faces isn't a rash, necessarily, but more like the red mark left over after removing a band-aid or a sticker from the skin. The cross was there all along, but it was invisible, some sort of spiritual mark put on humanity to signify that their sins are forgiven. When Jesus decided to take it back, he peeled that cross off their faces and thus the mark was left. (Fridge Brilliance sets in when you remember that the rash seems to start at the edges of the face and move inward, just like a sticker being peeled off.)

Naturally, the state of being unforgiven and having the sacrifice revoked means that mankind resumes their sinful ways, and then you suddenly realize why God felt the need to flood the Earth and sacrifice his son to save these creatures.

The Crossed seek out non-Crossed in order to afflict them with the same curse, because this is part of the programming Jesus left imprinted on their souls. This is why they also speak in red font. In The Bible, Jesus' parts are often written in a red font. Or, maybe...
  • A major problem with this theory is that it assumes that the Cross is an exclusive Christian symbol or one chosen by Christ. Its not. Early Christians used a fish as a symbol and the Cross came from Constantine, so its not necessarily indicative of Christianity except in a strict cultural sense.

The Crossed are possessed by Jesus Christ.
The cross is his calling card of sorts, the blood/saliva/other fluids is how he enters the body. The red font is Jesus' voice. As for why Jesus is desecrating everything? See above or, possibly, this is what his forgiveness is SUPPOSED to look like.
  • All of man's sins are forgiven, therefore there is no guilt. Shorn of guilt by Christ's love, humanity acts on all the impulses we normally repress.