Unlike all living organisms, most viruses have genomes that consist of single-stranded (ss) RNA rather than double-stranded (ds) DNA. These ssRNA viruses replicate within hours and evolve faster than any organism; their genomes mutate rapidly because--unlike dsDNA--ssRNA replication does not involve proofreading corrections. However, the less common dsDNA viruses enjoy another unique feature: The pressures in them have been found to be as high as 50 atmospheres (1-3)--more than 10 times the pressures found anywhere else in a living system. How do these high pressures arise, and what roles do they play in viral infectivity?