A method for using thiazine dyes, especially methylene blue, alone or in combination with low levels of light, to treat parasitic diseases is described. Examples of useful thiazine dyes are methylene blue, azure A, azure C, toluidine, and thionine. The preferred dye is methylene blue, administered orally twice a day. Since methylene blue absorbs in the red wavelengths, i.e., approximately 670 nm, which penetrates tissue much better than other lower wavelengths, light penetrating the skin to the capillaries at the surface can be used to enhance the activity of the dye. The thiazine dye can be provided in combination with other known antibiotics, anti-inflammatories, anti-parasitics, antifungals, and antivirals.