The present invention relates to a devices and methods for killing and/or debilitating pathogenic microorganisms in or on a patient's body, such as light-sensitive bacteria including helicobacter pylori and/or propionibacterium acnes. A chemiluminescent light source provides electromagnetic radiation having predetermined wavelengths in the visible spectrum. The light wavelengths are selected for absorption by naturally-occurring photosensitive chemicals produced by the patient's body.