Many bacteriophages are virulent. Virulent bacteriophages invade host cells and lyse the host cells to release progeny bacteriophage particles. Not all bacteriophages are virulent.
Pseudotemperate bacteriophages form an unstable carrier state within host cells.
Replication of a pseudotemperate bacteriophage is repressed most of the time, and the host cell continues to grow and divide. Pseudotemperate bacteriophages have been described for several types of bacteria. Pseudotemperate bacteriophages generally contain large genomes. This observation suggests that these bacteriophages may contain genes that are dedicated to functions other than bacteriophage replication and structure. Indeed, at least some types of pseudotemperate bacteriophages significantly alter host cell physiology during the carrier state.