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Association between known virus families and the presence of a cell wall, surface layer (S-layer), or absence in the hosts they infect. The schematic phylogenetic tree represents our simplified taxonomic classes as defined in the text. The abbreviations for the different host classes are as follows: Ba, Bacteria; Ar, Archaea; Pl, Plants; Pr, Protozoa; Fu, Fungi; Inve, Invertebrates; Ve, Vertebrates.
