"Version: 20171201"--Title page verso."A Morgan & Claypool publication as part of IOP Concise Physics"--Title page verso.Includes bibliographical references.1. Introduction -- 1.1. The Janus face of photomedicine -- 1.2. Photodynamic therapy--the killer -- 1.3. Cellular effects of PDT -- 1.4. Photobiomodulation--the healer2. Cancer stem cells -- 2.1. Evidence for the existence of CSCs and their…