Abstract: Assays and diagnostic devices for rapidly distinguishing between bacterial or viral sources of infection in a biological sample from mammalian subject are provided. A biological sample from said subject is contacted with a diagnostic reagent that detects at least one, or a pattern of multiple, volatile organic compounds (VOC). Detection of the VOC or a pattern of said VOC indicates a single source of infection selected from a Gram-positive bacterial infection, a Gram-negative bacterial infection, or a viral infection. In one embodiment, a dipstick diagnostic device permits rapid discrimination between bacterial or viral infection based upon the reaction of a detectable VOC reactive compound and optional label positioned on the dipstick surface.