In the shaman's eyes illnesses are due to only a small number of causes:
one of the most frequent is an extraneous object intruding into the body (intrusion).
In such cases, the symptoms are more physical than mental, unless the intruding object
is found in the head or the chest.
An alien object entering the body may sound weird to Westerners. Even stranger, these objects may be bone fragments, glass pieces, iron nails, hairy caterpillars, spiders, scorpions, bugs and beetles, reptiles and also fish. Actually the object is often not material at all but it has a definite spirit identity whose appearance reveal its specific power. So if I see a spider intrusion in a person's body, it means that the object shares the spider's power: an insight into the spider's features will show the nature of the disease.
Orthodox medicine too, is based on the belief that many illnesses are due to extraneous beings
entering the body (namely bacteria and viruses).
It's an analogy, but not an equivalence: when a shaman extracts an intrusion, he/she does not extract a material bug. Some state that they extract the microorganism's "life essence" (or energy or soul). It may be so. Yet some diseases that science does not ascribe to external agents are viewed as intrusions by shamans.
We can suppose that science hasn't yet discovered the disease-producing agent in such cases - remember the common cold, which is due to a virus and was once believed to be caused by low temperatures.
It is amusing to note that from the shamanic viewpoint, peptic ulcers have always been due to an intrusion, whereras Western doctors thought differently for centuries - until recently, when they discovered that it's caused by an extraneous bug, called Elicobacter pylori.
Nonetheless some spirit intrusions don't seem to be connected to anything material: e.g some extraneous objects which may cause depression or bad luck... We should think it over, maybe.
Anyway we shouldn't forget that shamans and physicians look into different levels of reality.
To regard an illness as a crustacean or reptile or fish intruding into the body isn't as exotic as it looks. An example is cancer. The first scientists who studied it gave it that name, meaning crab, because it looks like a crab as it spreads. They could as well call it spider, the appearance is analogous, maybe it looked like a crab in their eyes: they "shamanically" perceived the power of crab in that entity. Further examples are polyp and lupus (a Latin word for wolf).
The world where we belong is called Middle World by shamans and it is inhabited by spirits too.
Intrusions are elementary beings, which aren't bad themselves, but having an energy too alien to ours just
like the beetles and bugs whose aspect they share.
The more dangerous and large entities live in far recesses of the Spirit World, so that we seldom venture to meet any, except when emotional storms bring them to the surface - just as big rainstorms bring out into the open huge insects and spiders, that we have never seen before because they usually dwell in secluded places.
Wild environments being altered, houses being build or demolished, emotional disorders in our or other people's lives or in animals living nearby, especially big sorrows because pain affects us more deeply than other emotions do -- all such events let loose the spiritual equivalent of caterpillars and bugs who may intrude our own bodies if we aren't aware and well-protected.
Some other times intrusions are sent by Spirits or by an enemy shaman.
As to the first case, we share our world with plant spirits, animal spirits and place spirits (such as the spirit of a lake or a mountain), plus the Powers of Directions, Wind, Fire and the souls of the dead: each of them may hate us for some reason.
E.g., souls of animals killed without a cause or with cruelty or spirits of places disturbed or offended may wish to punish us and make us face our own mistakes.
At times a spirit may attack us only because its own interest conflicts with ours: it's analogous to material bugs and parasites which live on damaging their hosts.
As to attacks by hostile shamans, these are not as rare in our civilization as you might think.
Some people are "natural sorcerers" who are able to perform spontaneously what shamans do out of their knowledge.
The shaman wanting to harm enemies shoots a magical dart,
at them. The energy of the magical darts comes from the shaman's personal Powers, especially from the allies,
which are often held in the stomach. I'll write about this in the article on
The dart sticks in the victim's body causing chiefly physical diseases, but sometimes depression or misfortune. Btw, as for depression and misfortune, it's hard to tell which causes which...