Final answer:
Virulent phages are bacteriophages that infect bacteria and replicate exclusively through the lytic cycle, causing the lysis and death of the host cell. Unlike temperate phages, they do not integrate into the host's genome to enter a lysogenic cycle.
Step-by-step explanation:
Bacteriophages that cannot undergo lysogeny but can infect bacteria are called virulent phages. These phages are known to only engage in a lytic cycle, which is characterized by the replication of the virus within a host cell followed by the lysis and death of the host. Virulent phages, such as T4, which infects Escherichia coli, do not integrate their DNA into the host cell's genome, a process associated with temperate phages during the lysogenic cycle.
A virus such as the lambda phage is an example of a temperate phage that has the potential to incorporate its genome into the host's DNA, becoming a prophage and enabling the bacterium to reproduce with the viral DNA integrated. This state of integration, known as lysogeny, can be induced to leave by environmental stressors, causing the prophage to excise from the genome and revert to a lytic cycle.