Abstract: Method and compositions for infectious disease diagnosis and epidemiology involving labeled nucleotide probes complementary to nucleic acid coding for a characteristic pathogen product. Clinical isolates are cultivated, expanding the number of microorganisms, the resulting colonies lysed, the genome normally denatured and then fixed. Alternatively, clinical samples (stool, sputum, pus, etc.) are spotted onto an inert support. The sample is treated in such a way that the DNA is liberated from microbes present in the sample and complexed onto the support. The DNA is normally denatured and fixed in this process. Subsequently, a labelled polynucleotide probe specific for a DNA sequence characteristic of a pathogenic product suspected of being present in the clinical sample is contacted with the fixed genomic single stranded nucleic acid under hybridizing conditions. Hybridization of probes to the single stranded nucleic acid is diagnostic of the presence of the pathogen.