Varicella zoster virus (VZV) is the virus responsible for two distinct diseases: chickenpox on first infection and shingles if it reactivates later in life. It belongs to the herpesvirus family and has a unique ability to hide in your nerve cells for decades after the initial illness, potentially re-emerging when your immune system weakens. Before vaccination became widespread in the United States, VZV infected more than 4 million people every year.
How VZV Spreads
VZV is highly contagious and spreads through multiple routes. You can catch it by breathing in tiny particles released from the skin blisters of someone with chickenpox or shingles, through direct contact with blister fluid, and possibly through respiratory secretions that become suspended in the air. This airborne capability makes it one of the more easily transmitted viruses, especially in enclosed spaces like classrooms or hospitals.
A person with chickenpox becomes contagious one to two days before the rash even appears and stays contagious until every blister has crusted over. Someone with active shingles can also transmit the virus to a person who has never had chickenpox or been vaccinated, though in that case the newly infected person develops chickenpox, not shingles.
Chickenpox: The First Infection
When VZV enters your body for the first time, it causes varicella, commonly known as chickenpox. The incubation period averages 14 to 16 days, though it can range from 10 to 21 days after exposure. The hallmark is an itchy rash that starts as small red spots, progresses to fluid-filled blisters, and eventually crusts into scabs. Most children recover without complications, but the illness tends to hit harder in teenagers, adults, and anyone with a weakened immune system.
Pneumonia is one of the more serious complications, estimated to occur in roughly 1 in 400 chickenpox cases overall. Adults face significantly higher risk. In one study of military recruits who developed chickenpox, about 16% showed signs of pneumonia on chest imaging. Pregnant women developed this complication in approximately 9% of cases. These numbers help explain why chickenpox, often dismissed as a mild childhood illness, was responsible for 10,500 to 13,500 hospitalizations and 100 to 150 deaths each year in the United States before the vaccine was introduced.
How the Virus Hides in Your Nerves
After chickenpox resolves, VZV doesn’t leave your body. Instead, it travels along nerve fibers and settles into sensory nerve clusters called ganglia, which run along your entire spinal column and into your skull. There it enters a dormant state called latency, where it essentially shuts down its normal replication machinery. The virus suppresses the genetic program it would normally use to produce new copies of itself, keeping a very low profile inside nerve cells.
This dormancy isn’t completely silent. Certain regions of the viral genome remain mildly active, producing small amounts of specific proteins. Researchers believe latency involves an ongoing balancing act of suppression and low-level gene activity, rather than a total shutdown. This delicate equilibrium can persist for decades without causing any symptoms.
Shingles: When the Virus Reactivates
When VZV breaks out of dormancy, it causes herpes zoster, better known as shingles. The virus travels back down the nerve fibers to the skin, producing a painful, blistering rash. Unlike the widespread rash of chickenpox, shingles typically appears as a band or strip on one side of the body, following the path of the affected nerve. Common locations include the waist, chest, back, neck, and face. The rash doesn’t cross the body’s midline, which is one of the key visual clues that distinguishes it from other skin conditions.
The blisters usually form three to four days after the rash first appears, then dry out and scab over within about 10 days. But the pain can start before any visible rash and may feel like burning, stabbing, or deep aching in the affected area.
The primary risk factor for reactivation is age. Your immune system naturally weakens over time, and shingles risk rises substantially after age 50. People with compromised immune systems, whether from conditions like HIV or cancer or from immunosuppressive medications, face elevated risk as well. Stress may also play a role, though this has been difficult to confirm definitively.
Postherpetic Neuralgia
The most feared complication of shingles is postherpetic neuralgia (PHN), a condition where nerve pain persists long after the rash has healed. For many people, this pain lasts months or even years. In some cases, it never fully resolves. The pain can range from a constant burning sensation to sharp, shooting episodes triggered by light touch on the affected skin. Diagnosis is usually straightforward based on a history of shingles followed by ongoing pain in the same area.
How VZV Is Diagnosed
When a doctor needs to confirm a VZV infection, the most reliable method is a PCR test, which detects the virus’s genetic material in samples taken from skin lesions. Ideal specimens include fluid from blisters, scabs, or cells scraped from the base of a lesion. PCR works quickly and with high sensitivity for both chickenpox and shingles, and it’s reliable in both vaccinated and unvaccinated individuals. Older techniques like viral culture and direct fluorescent antibody testing exist but are less sensitive and generally not recommended.
Vaccines and Their Impact
Two vaccines target VZV at different stages of life. The varicella vaccine, introduced in the United States in 1995, protects against chickenpox. The results have been dramatic: chickenpox cases have dropped by more than 97%, from over 4 million annually to fewer than 150,000. Hospitalizations fell from more than 10,500 per year to under 1,400, and deaths dropped from as many as 150 to fewer than 30. Over its first 25 years, the vaccination program prevented an estimated 91 million cases and saved $23.4 billion in healthcare costs.
For shingles prevention, the Shingrix vaccine is recommended for adults 50 and older. In people aged 50 to 69 with healthy immune systems, it is 97% effective at preventing shingles. That effectiveness dips slightly to 91% in adults 70 and older. Protection against postherpetic neuralgia is similarly strong: 91% in adults 50 and older, and 89% in those 70 and above. For people with weakened immune systems, effectiveness ranges from 68% to 91% depending on the underlying condition.
The Virus at a Biological Level
VZV is a member of the herpesvirus family, specifically the subfamily of alphaherpesviruses. Its genetic material is linear, double-stranded DNA, organized into two main coding regions flanked by repeated sequences that help regulate the virus’s activity. This genome structure is typical of its close relatives, which include herpes simplex viruses. VZV is classified in the genus Varicellovirus, reflecting its role as the cause of varicella. Despite being one of the smaller herpesviruses, it carries all the genetic instructions needed to infect cells, evade immune detection, establish lifelong latency, and reactivate under the right conditions.