Physical virology presents a new direction that aims to provide a physical description of common mechanisms controlling viral replication for a broad range of viruses. Releasing the packaged viral DNA into the host cell is an essential process to initiate viral infection. In many double-stranded DNA bacterial viruses (phage) and herpesviruses, the tightly packaged genome is hexagonally ordered and stressed in the protein shell called the capsid. DNA condensed in this state has been shown to have restricted fluidity. It has remained unclear how this rigid crystalline structure of the viral genome rapidly ejects from the capsid, reaching rates of 60,000 base pairs per second. Through a combination of single-molecule and bulk techniques, we determined how the structure and energy of the encapsidated DNA in phage lambda regulates its fluidity required for ejection. Our data show that packaged lambda-DNA undergoes a solid-to-fluid disordering transition as a function of temperature, which facilitates rapid genome release at temperatures close to that of viral infection (37°C). This suggests a remarkable physical adaptation of bacterial viruses to the environment of E. coli cells in a human host.