Spiritual intrusions can be caused by anumerous things.
Toxic energy, psychic darts, spiritual parasites and even curses.
Spiritual intrusions are not Physical but will most likely manifest as a physical symptom if not cleared.
One of the first steps to clearing an intrusion would be to call upon your spirit guide/helper asking them to assist you in finding the source and it's origins..
There may be many different symptoms that indicate an intrusion, localized pain, discomfort, tightness or pain in the shoulder area (this would be know as a dart someone has thrown at you in anger) obsession of a thought form resulting in a psychic mass and debilitation in ones every day functioning.
One of the first steps to preventing an intrusion would be to learn to shield from external attacks and any would be anger directed towards you .
One may also carry power objects, small carving of totems or crystals(galena, hematite, granite are all very good for absorbing negative energies.(moldavite transforms negative to positive energy)however if you are an extremely sensitive person this crystal may be to overwhelming.(selentine can also transform negative to positive specifically used by many healers.(Black obsidian is a good all purpose protection crystal.
1:Find a quiet place in nature woods, park, can even be your own back garden if you wish.
2:Remove ones shoes and plant feet firmly on the ground Keeping your back straight and head up bending knees slightly so as not to become rigid.
3:Breath calmly and rhythmically closing your eyes call upon your spirit guides and totem animal(s) to assist you, trusting that whomever you call will be the right ones to assist you.
4:Now begin breathing slow and steady, visualizing a pure clean water flowing down from the top of ones head(crown) and slowly moving down every part of your body, visualize this clean pure water moving through every bone, tissue organ. Become aware of how this water is taking away every impurity With each breath.
5:Feel the water releasing all of the toxins (flushing) them down through every part of your body and out into mother earth draining away all the impurities.
6:Repeat the process until the water you have visualized runs completely clear.
7:Thank your guides/totems and mother earth.
8: visualize a pure white light filling every part of you. Begin to become aware of your surroundings once more and open your eyes.
You have now completed your first clearing intrusions.