The reasons why it occurs, psoriasis, can be varied because it is a multifactorial condition and no single cause can be determined with certainty although in many cases the genetic component plays a role.
Psoriasis is a chronic inflammatory disease of the skin that manifests itself as thickened areas covered in scales that are a whitish colour fading to silvery-grey. In some cases, these lesions can cause itching. Psoriatic plaques usually appear on the elbows, knees, scalp and lower back, but can also extend to the entire body.
Psoriasis is not contagious, so being near or touching a person with psoriasis does not in any way lead to transmission of the disease.