The most common form is Idiopathic Parkinson's disease, which is termed thus since the cause is not known, whereas the cause is either known or at least suspected in its other forms, such as Postencephalitic Parkinsonism, Drug-Induced Parkinsonism, Striatonigral Degeneration, Toxin-Induced Parkinsonism, Arteriosclerotic Parkinsonism or Pseudoparkinsonism.