To the Shaman, there are 3 common causes of illness: loss of power; loss of part of the soul or essence; or negative energy blockages.
Loss of power might cause chronic illness, depression, or general bad luck. Someone who's lost part of their soul or essence, possibly through an accident, surgery, addiction, divorce or even war might be similarly afflicted. And negative energy blockages - even spirit possession - can often be due to the soul or power loss, and can cause illness in a particular area of the body.
With shamanic work, the practitioner is said to journey to different dimensions to retrieve the clients' power or soul fragments and to clear blockages in the spirit.