(Only speaking from Northern Tradition)
Shaman sickness is defined in this tradition as being a period of illness that is provoked by either the Gods or the wights (or both) to draw a person to shamanism, and re-make them, so that they are capable of doing some work associated with this practice.
We divide shaman sickness into two main "paths":
- The Path of Madness
- Hel's Path
These paths go by different names, but those are the ones I'm most familiar with using. I want to make it clear that it is entirely possible to walk both of these roads, either simultaneously or otherwise. These roads can cross, and sometimes involve aspects of both. This isn't something that occurs to everyone who practices (Raven K suggests the large majority of people who practice do not experience this), but it does occur and is a big part of the practice I am involved in regardless. This doesn't mean that anyone who has had a close-call or a mental illness is a shaman or ever will be, but it isn't uncommon for Northern Shamans to have walked with or experienced one of the these two paths- and most who have experienced these two paths and decided to continue or deepen their workings in Northern Tradition shamanism view this experience as a "calling" to do their work.
He'ls Path is known as The One Road and also Death's Road. This is the physical manifestation of shaman sickness, wherein the body is attacked. It's said that most who end up traveling down this path will endure near-death experiences, some type of chronic illness, long drawn out sicknesses that don't respond well to treatment, a lowered and weakened immune system, experiencing a traumatic and serious physical injury, or an actual death. Keep in mind, however, it isn't always an extreme sickness and the person who undergoes it might not reach those depths of sickness nor experience close calls with depth. I personally think that it really does vary from individual to individual and everyone will experience it different first-hand.
I've heard it suggested that those who walk down Hel's path often change after the experience, not only in spiritual and physical re-birth, but in other things such as their pathwalking forms and their sheer presence. They're often described as creepy or unsettling to be around, because those who sense auras and whatnot describe them as "smelling of death" or giving off a death-like vibe. That's mostly attributed to the fact that on these paths, part of the practitioner is believed to never come back (this part of the person is called Hame, but this comes from older Norse concepts of the soul- which you may want to look into so you understand what this is talking about). So I suppose they are still somewhat in that state, and that would be why those feelings and whatnot are experienced. I assume it would feel creepy or uncomfortable to someone who's never died or been that close to death, as that is a foreign feeling.
It's important to keep in mind, I think, that some people may not be familiar with some of the rituals and traditions practiced in this type of cultural shamanism I am speaking about, so to clarify on something: Some rituals and workings do require a person to purposefully face death-like experiences, though it doesn't encourage doing any sort of work like this without proper medical attention. It comes in at varying points in the practice, such as in the altered states of consciousness where pain and deprivation are used to achieve certain trances and alteration, when working with Hel, and it comes in to play in other practices like "Going Under" or "Sitting Out" and other various workings.
*Interestingly enough I just learned not too long ago when I got sick that Raven K, my idol, walked down Hel's Path. Shortly after Hel took him into her arms for his practice, he developed lupus and secondary congenital adrenal hyperplasia, in which he literally died once. Completely blew my mind to hear this! Was kind of excited in a way to know Raven deals with some of the same illnesses that I have faced myself.*
Of course, getting medical treatment is a must if someone is experiencing sickness to that degree. However, often they are urged to seek other forms of treatment as well as the professional medical kind. They are urged, in this practice, to do pathwalking and spirit-work and use forms of divination to try and find the ultimate cause of this physical sickness. Some of the practitioners I personally know described this as the spirits "destroying their body and rebuilding it to make them new and capable". In some of the rituals you symbolically act this out, the death and rebirth, to complete some sort of task.
The Path of Madness is often called The Other Road and just Madness. Instead of a physical sickness or death, this is seen as a sort of death of the mind and/or the personality of the person before the path is walked. Aka, mental illness. As far as I know, this tends to be the more common of the two- at least in Northern Tradition.
This can manifest in a large variety of ways. Sometimes it can be seen as the deadening of a person's interests and their spirituality, they lack the luster for life and become almost like "the walking dead", which is why in this practice they are sometimes called "Dead men walking". Sometimes it can be seen as suddenly "hearing voices" or otherwise pathwalking and projecting without understanding what is happening, hallucination or seeing visual images that don't make any sense. Schizophrenia, depression, bipolarity and other mental illnesses are sometimes carried by those who walk this path (again, stressing that just because you have a mental illness- does not mean you are being called to shamanism). Northern tradition shamans tend to see this kind of sickness as the person being called to pathwalk and journey, if they have never done so before, and their mind trying to adjust to it.
There's kind of a debate on this path about the use of medications to help some of the symptoms. On the one hand, some people may truly need the medication- else they end up in the hospital or worse. On the other, in some practices you actually are supposed to walk this path to achieve some end result- and in that case many strongly feel that you need to experience this path fully to understand it. In either case, to be safe with any type of practice especially this one I would seek medical help- just because you have some mental illness does not mean you are walking down this path, and a doctor will be able to help you regardless.
I like what Raven K said about it:
" While one might think that the Madness Road is, if not easier, at least less life-threatening than the Death Road, that would be incorrect to assume. A spirit-worker on the Madness Road may hurt or otherwise injure out of pain and despair, or do something stupid that gets them killed, or go so thoroughly mad that they burn out their own gifts and live practically catatonic for the rest of their (usually short) lives. One of the big dangers of the Madness Road is being too crazy to realize that you're all that crazy, especially if you've actually got wight-contact going at the same time. It's also common for your judgment to be entirely off about all the important things in your life, even the simplest ones. "
So that's what I've learned from my own practice, but I would be very interested in comparing it to other traditions or cultural practices and beliefs as well.
Sources:
http://www.virtualcs.com/dsm4/lesson3_8.html
http://www.tengerism.org/becoming_a_shaman.html
http://www.crowcity.co.uk/mad/skzshamn.html
http://shamanism.wordpress.com/2007/09/22/death-as-an-ally-the-shamans-perception-of-death/
http://www.nor.com.au/community/spiritualemergence/page17.html
Wyrdwalkers: Techniques of Northern-Tradition Shamanism by Raven K