Diagnostic nucleic acid hybridization probes are constructed from the most conserved portions of genes from viruses commonly causing infection. Long probes have a large inherent tolerance to microbial variation. The introduction into the introduction into the probe design of a base that can hybridize with all four normal bases (a universal base), or of multiple nucleotides (degenerations; wobbles) in a single position, can induce tolerance to natural viral variation (mismatch).