This affliction is caused by a living spell. It lodges in the infected creature, usually in the brain or other energetic center, where lots of energetic pathways intersect.
In stage 1, the spell is seedlike, leeching energy from the surrounding pathways. Eventually it grows in a small, yet magically contained, fireball, which grows larger and larger. The host may experience tiredness and occasional confusion, however the effects in this stage are subtle.
In stage 2, the fireball has grown to about 3% of the hosts overall size. The host begins to manifest symptoms, which vary by species, but often include headaches or other body pain, confusion or erratic behavior, dry skin or hot flashes, and frequent complaints of feeling cold. The spell is leeching the head and energy at a most advanced pace in this stage.
Stage 3 comes on rather suddenly. Once the fireball reaches 5% of the hosts overall mass, it doubles in size every 5 hours until it reaches 30% of the hosts size. This process is devastating to the host, who generally loses all conscious will. The skin becomes gaunt and sunken as huge amounts of vital energy are absorbed by the fireball, and it turns an ashy grey color. The host becomes a shambling, clumsy zombie, only barely aware of its surroundings and largely consumed in a fog of pain and numbness. At this stage, the fireball is rapidly creating tiny replicas of itself and storing them in a special magically insulated extradimensional space at the center of the spell. There are also other strange secondary magical effects; rather than bursting through the hosts skin, as you would expect, the skin dries out and stretches. In fact the host is no longer traditionally alive at all; the magic of the fireball is partially used to maintain a tenuous connection to vitality and movement. The fireball itself uses a very small part of its magic to levitate, allowing the host to move despite the large volume bulging out of its head (at least in the case of bipedal creatures).
The end result is a zombie like, shambling creature which wanders around, seemingly at random, until the fireball has created many thousands of replicas, and is at maximum energy output. If the host doesn't contain enough energy natively to facilitate this process, then the fireball will obtain more by using the host body to consume other life forms; plants, animals, and even carrion will serve, given enough volume, and better yet magical items, gems, or other sources of energy can be drained through a sort of siphoning process that resembles spiritually drinking through a straw.