Valor's Rot

Listen, I wouldn't go into the medical tents if you can avoid it. It's never pleasant and you'll never get what remains of those guys out of your shoes.

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Virtus Putrescare, commonly known as Valor's Rot or the "soldier's disease" is a condition that develops within those that have little to no magical inclination that are exposed to ambient particles of magic and magical ingredients in the air around them.
The disease starts as a wet cough that, if left alone, will heal on it's own though the subject would need to leave the general area that was damaging them for a significant time, something that is not always easy for those that live in magically dense areas, or pollution by material. Should the disease progress, however, the cough will continue to worsen until blood begins to travel up with each cough. At this point, the subject should be put into hospice.
To explain the biology of Valor's Rot, one must look to the internal magical flow of the individual in question and how the disease messes with it. Over exposure causes many of the channels that magic flows through within the body to close and shut down. As this happens, magic starts to run rampant throughout, looking for a place to go, dealing damage the whole way. Nothing is safe from the damage. Organs degrade first, including heart and lungs, resulting in the aforementioned blood-filled coughing. The general flesh of the body follows soon after.

Type

Bacterial

Suppose one follows the currently held belief of what causes Valor's Rot. In that case, one may come to an uncomfortable conclusion, that being that even being magically inclined isn't a perfect defense against the disease. Since the disease affects those with even a little magical control, it stands to reason that there's an amount of ambient magical energy that would affect even the most powerful magus.
That being said, it should be less of a problem for those trained in magic as they would be able to express and organize the magic within themselves.