be accurately identified by combinations of a series of simple tests such as morphology, staining, antibiotic sensitivity, biochemical analyses, nutrient dependence, and phage sensitivity. Viruses, however, have proved much more difficult; their size and absolute dependence of the host cell for propagation have rendered useless the methods traditionally used for other microorganisms. Until the development of tissue culture in the middle of this century, diagnosis was entirely dependent on the skill and experience of the clinician. But this was an unreliable process since many of the common virus infections exhibit similar clinical symptoms, such as coryza, exanthema, vomiting, diarrhea, neuralgia, and lethargy. Indeed many viral infections display clinical signs that are indistinguishable from bacterial or parasitic infections.