Sentence:  `` Unfortunately '' , says Chief Postal Inspector David H. Stephens , who has prosecuted many device quacks , `` the ghouls who trade on the hopes of the desperately ill <WSD>often</WSD> cannot be successfully prosecuted because the patients who are the chief witnesses die before the case is called up in court '' . 
Meaning of the word marked with <WSD> tags:  many times at short intervals