A method of selecting an exon of an RNA whose expression level is informative with respect to infection type of a subject is disclosed. The method comprises comparing the expression level of the RNA in a sample derived from a bacterially-infected subject and a sample derived from a virally-infected subject at a plurality of exons, wherein the exon that provides a differential expression between the bacterially-infected subject and the virally-infected subject above a predetermined level is selected as the exon of the RNA whose expression is informative with respect to infection type.