a class name proposed earlier for the viruses and rickettsias, infective agents that grow and reproduce only inside living cells. The order Virales comprises the viruses of bacteria, plants, and animals. The order Rickettsiales comprises the rickettsias, somewhat larger parasites, the growth cycle of which includes stages within both arthropods (e.g., insects and arachnids) and vertebrates. Although the class has been abolished, the ordinal names survive.