The present invention relates to methods of detecting an infection in a subject based on the relative abundance of target molecules indicative of the expression of at least the gene pair DUSP1 and IFI27. In particular, the invention relates to a method of distinguishing between a bacterial infection or a non-bacterial infection in a subject, especially between bacterial and non-bacterial lower respiratory system infections using the gene pair. Further, the invention provides the medical use of therapeutic agents in the treatment of a bacterial or non-bacterial infection in a subject identified as having such an infection through use of a method of the invention, and monitoring the therapeutic effectiveness.