Fading

Fading is a name for a sneaky condition that slowly takes away your life from the inside. While it is mostly associated with the elderly, it seems as though it can affect anyone - although it is very rare to manifest in children unless they have gone through some serious trauma.
Scholars of the nature and the body agree that your mind and your memories are the first to go, hence the name. It looks like a case of dementia at first, the way people start forgetting very simple everyday things, like where they put their equipment, or how to cook certain things, or the names of their family members or colleagues. This disease starts to eating away all the things that are powering you through the every day life. The it comes for more long term memories associated with your past and childhood, while there is a specific type of coldness spreading all throughout your body.
Your head starts to feel heavy, then the breathing gets more laboured and it can feel like you are not really able to feel warmth ever again. Then goes your appetite and movement of your upper limbs. This is when a lot of the sick people get injured because, surprisingly, their legs are working just fine. However, their mind is long gone, they are only able to use limited speech and they are easily confused and frightened. A lot of them try to run away in confusion or fear and thus get injured from falls or they get completely lost and befell onto some dangerous and hungry beasts in the wilderness. 
It is not always possible to keep the sick in confinement to keep them safe. Although the further deterioration of their bodies seems to follow fairly quickly. It is usually only a matter of couple of weeks from the victims starting to have problems with their bowel movements to their legs also becoming immobile and eventually passing away in their sleep.
Some people who are aware that they might be suffering from fading choose to end their lives on their own terms to prevent their friends and family from seeing them fade away in such a manner.