Spirit entities as well as energies regularly attach themselves to human energy fields. It tends to be the spirits of loved ones who did not pass on properly after death. Sometimes it is quite a dramatic experience when you release one of these from the field. Other times it is simple and fast. I had somebody remove my grandfather from my field a few years ago. The shaman worked over the phone, and when the phone kept hanging up and I got through to CHINA, I knew that it was time to let this entity go.
There are many ways to remove an entity. Christians call it exorcism, shamans call it extraction. The easiest way to remove an entity is with the use of a crystal, but it is vital to process the extraction in the chakras to remove the affinity to the entity/energy.