Concept Art 61: The Corrupted.

Corruption is known in Arcadia as a sickness originally found in powerful witches or sorcerers who gave in to their ambition for more power, and as holy punishment for abusing God's gift (magh), they slowly lose any trace of humanity until all that is left is a horribly deformed creature.

As such illness it is thought to be as contagious as the flu, be it by the mere contact with a corrupted one, a bite, wound, or a direct transmission of fluids. Some even think that their breath or mere presence is enough for infection.

As you can see Acadians have a very fantastical and fear-mongered idea of what corruption is, yet the truth of its origins is a bit more complicated. For starters, corruption came to be from different sources: Adam, Syzygy, and the Exicom Empire.

As for Adam as the source, it is possible that the corruption could be either an intentional or accidental genetic mutation present in the DNA of the Ixim. This mutation, if triggered, could cause the glands that produce magh to overdevelop and produce exaggerated levels of Magh which ultimately would be too much for the host to contain in their original form, thus their form starts to mutate accordingly making the subject start to show physical similarities to the Reapers, which are entities made of pure magh. The ixim and  Reapers aren't as different as you might think, In the end, both of them are products of Adam's blood (magh), one is mixed with ancient human genetic codes and the other is just but the manifestation of pure magh, so the more an ixim starts to produce magh the more it starts to get closer to purity and thus look like a Reaper. It is also a possibility that Adam could trigger a mutation in any ixim at will if so he desires.

What triggers the mutation? To be honest, it could be caused by many factors, it could be just that some individuals are more prone to it than others, although Arcadians might be on to something when they think that too much magh triggers it. For example, being exposed to a large amount of Magh during extended periods could have a similar effect as radiation and trigger the mutation. So if a witch or sorcerer were looking for unusual ways to get stronger, it's not out of the table to think that could lead to corruption.

As for syzygy, the AI used many ixim subjects as guinea pigs for her research on Magh and how to take advantage of it, ultimately her experiments led to failures (uncontrollable corruption) and  Triumphs as the Nephilim, which have a mutated ixim as the core. Most of the failures were discarded or thrown out in Arcadia.

In the Case of the Exicom empire, if you have read  Belias's backstory I do mention his involvement in the development of a virus that could trigger the mutation. Although his investigation was discarded shortly after seeing the collateral damage that it caused, later on, King Frederick would resume the experiments to try to contain divinity (reapers) in Human vessels that he could control for his own gains.

As a side note, Resident Evil and Silent Hill inspired these designs!