Given the compact sizes of viral genomes, only few viral proteins fall into the category of bona fide virulence factors, similar to the potent bacterial toxins discussed above. By contrast, most viral proteins are dedicated to basic processes essential to the virus life cycle, such as entry or exit, replication, or manipulation of host processes such as transcription or translation. Model organisms are useful for examining specific interactions between viral and host proteins to gain insights into their mechanisms of action.