Similarity scores for rabies virus were low between Canis lupus familiaris (domestic dog) and Bos Taurus (domestic cow) ( 2 S i,1 = 0.280, 2 S i,2 = 0.373, and 2 S i,3 = 0.366) and also between B. taurus and H. sapiens ( 2 S i,1 = 0.267, 2 S i,2 = 0.371, and 2 S i,3 = 0.416) but were high between C. l. familiaris and H. sapiens ( 1 S i,1 = 0.947, 1 S i,2 = 0.985, and 1 S i,3 = 0.962). Clearly, C. l. familiaris is the primary reservoir, and transmission of the disease to H. sapiens is possible only because of the high human/ dog receptor similarity. Thus, for particular viruses, transmission of disease may be species-selective, although common receptors exist among species. Furthermore, infection specificity may be determined by the species barrier, which results from receptor differences.