Somehow this idea came to me. I don't know how or why but it seems like it could be interesting.
This is a disease that technically doesn't exist. It is not caused by a pathogen or other physical condition. It is transmitted by a subject thinking he has the disease. The disease then enters a primary stage where mild but worsening random symptoms occur. These symptoms can vary between normal things like fevers, to unusual stuff like tissues being converted into other materials. However, these are all mild and will rarely do much other than confuse the subject. These symptoms cannot be treated and will often distress the subject, leading into the secondary stage.
After a certain length of time, the disease will enter a secondary stage where the subject's thoughts about the disease are translated into actual symptoms. Things like heavy fevers, brain damage, physical mutations, and especially death can occur. These symptoms persist, strengthening until a) the subject dies or b) the subject is convinced he has no disease.
Sandbox coming if the idea can work out…
