lytic and lysogenic cycles
/ LIT-ik and ly-suh-JEN-ik SY-kulz /
Once a virus gets its genes into a host cell, it faces a choice between two strategies, much like a burglar who could either smash-and-grab now or quietly move in and wait. The smash-and-grab option is the lytic cycle; the patient move-in option is the lysogenic cycle. These two named pathways describe what happens after infection, and many viruses can switch between them depending on conditions.
In the lytic cycle, the virus immediately commandeers the host cell's machinery to mass-produce new virus particles. The cell becomes a factory churning out copies until it is so full that it bursts open — 'lysis' means bursting — releasing a flood of new viruses to infect neighboring cells, and killing the host in the process. In the lysogenic cycle, the virus instead inserts its genetic material into the host's own chromosome and goes dormant. The hidden viral DNA, now called a provirus or prophage, is copied along with the host's DNA every time the cell divides, riding silently through generations. Later, a stress signal can flip it back into the lytic cycle.
These cycles matter for understanding how viral infections behave over time. The lytic cycle explains the rapid, destructive phase of an infection; the lysogenic cycle explains how a virus can lurk undetected for years and then suddenly reactivate — the pattern behind cold sores and chickenpox that returns as shingles. It also shows how viruses can permanently alter their hosts: a dormant viral gene tucked into a cell's chromosome can be inherited and can even change what the host cell does.
The virus behind chickenpox first runs a lytic-style infection that gives a child the rash, then hides for decades in nerve cells in a lysogenic-like dormant state. When it later reactivates — often when the immune system weakens with age — it erupts as the painful rash called shingles.
Chickenpox (lytic) hides for decades, then reawakens as shingles — a real-world lytic-to-dormant-to-lytic switch.
In the lysogenic cycle the virus does not kill the cell — it lies hidden in the cell's DNA. The classic textbook examples come from bacteriophages, but similar dormancy occurs in viruses that infect us, like herpesviruses.