In a shaman's journey, i.e. in a state of altered conciseness, a disease (physical, psycho-somatic, or mental) may present as a foreign object stuck in the clients body.
Shamanically, a disease is a piece of energy that does not belong to the energy of the client's body. This energy is neither bad nor good, it just is not in harmony with the rest of the person's energy.
In order to interpret this energy, and to know how to remove (extract) it, the energy presents as something tangible. I have seen it as spears or arrows, rocks, ticks, maggots and other, sometimes disgusting objects. The point is that the ways to remove the foreign energies depend on how they present to the shaman.
I have also made the experience that the type of object has to do with how it got into the client. Often, the object itself, while not really exceptional for the shaman, has a great meaning for the client.
In the extraction journey, the shaman not only locates the object, he/she also gets guidance on how to remove it, and what to do with it afterwards. This often results in a ceremony. Sometimes this ceremony has to be done on a certain day (e.g. solstice), under certain conditions (e.g. full moon,) or on certain places.
The actual extractions are often very hands-on ceremonies. I have experienced extractions that needed a big deal of labor and have caused nausea and exhaustion on my side.