Varicella zoster is a virus that causes two distinct diseases: chickenpox on first infection, and shingles if it reactivates later in life. It belongs to the herpesvirus family and infects only humans. After you recover from chickenpox, the virus never leaves your body. It hides in nerve cells near the spine and skull, where it can sit dormant for decades before potentially flaring up as shingles.
How the Virus Spreads and Causes Chickenpox
Varicella zoster is extraordinarily contagious. Up to 90% of non-immune people who are in close contact with someone infected will catch it. The virus spreads through airborne droplets and direct contact with blister fluid, and a person becomes contagious one to two days before the rash even appears. That window of invisible contagiousness is a major reason chickenpox spreads so effectively through households and schools.
After exposure, the incubation period runs 10 to 21 days, with about two weeks being typical. The classic symptom is a rash that starts on the chest, back, and face before spreading across the body. It progresses from flat red spots to itchy, fluid-filled blisters, and a single case can produce up to 500 of them. Over about a week, those blisters dry out and scab over. The whole illness usually lasts four to seven days. A person remains contagious until every blister has crusted.
How the Virus Hides in Your Nervous System
Once chickenpox resolves, varicella zoster travels along nerve fibers and settles into clusters of nerve cells called ganglia, particularly the dorsal root ganglia along the spine and the trigeminal ganglia near the base of the skull. There, it enters a dormant state called latency. The virus stops replicating and producing new copies of itself, but its DNA persists inside neurons. Researchers have detected viral DNA in these ganglia decades after the original chickenpox infection.
What’s striking is that this latency establishes itself even without help from the immune system. Your immune response doesn’t set up latency; it simply keeps the dormant virus in check. When that immune surveillance weakens, whether from aging, stress, illness, or medications that suppress immunity, the virus can wake up and begin replicating again. This reactivation is what causes shingles.
Shingles: What Reactivation Looks Like
Shingles typically begins with pain, burning, or tingling in a specific strip of skin on one side of the body. This pain can precede the rash by several days. The rash then appears as clusters of blisters along a single dermatome, which is a band of skin supplied by one spinal nerve. The trunk is the most common location, followed by the face. The rash almost never crosses the body’s midline, which helps distinguish it from other conditions.
New blisters continue forming for three to five days, then progressively dry and scab over. Full healing takes two to four weeks, though some people develop permanent scarring or skin discoloration. About one in three people in the United States will develop shingles at some point in their lives.
People with active shingles blisters can transmit the virus to someone who has never had chickenpox or been vaccinated. That person would develop chickenpox, not shingles. The virus spreads through direct contact with blister fluid or by breathing in viral particles from the blisters.
Postherpetic Neuralgia and Other Complications
The most common long-term complication of shingles is postherpetic neuralgia, a burning or shooting pain that persists for three months or more after the rash heals. It follows the same strip of skin where the rash appeared. Age is the strongest risk factor: roughly 13% of shingles patients aged 50 and older develop it. The numbers climb steeply from there. By age 60, about 60% of shingles patients experience postherpetic neuralgia, and by 70, that figure rises to 75%.
Other risk factors include severe pain during the initial shingles outbreak, a prodromal phase (pain before the rash), shingles affecting the eye area, diabetes, and severe immune suppression. There’s no difference in risk between men and women, and family history of shingles may also increase susceptibility. For most people, the pain does fade with time: about 5% still have it at three months, and 3% at one year.
When shingles involves the eye (a form called herpes zoster ophthalmicus), it can damage the cornea and threaten vision. In rare cases, the virus can affect internal organs or cause inflammation of blood vessels in the brain, leading to stroke.
Risks During Pregnancy and Immune Suppression
Chickenpox during pregnancy is uncommon, affecting roughly 0.5 to 0.7 per 1,000 pregnancies. But when it occurs, the consequences can be serious. Maternal illness ranges from a mild rash to life-threatening pneumonia. For the developing baby, infection during the first 20 weeks of pregnancy carries about a 2% risk of congenital varicella syndrome, a pattern of birth defects that can include skin scarring, limb underdevelopment, eye abnormalities like cataracts, and neurological problems. Premature birth is also more common, occurring in about 14% of pregnancies complicated by chickenpox compared to roughly 6% otherwise.
People with weakened immune systems, whether from HIV, cancer treatment, organ transplantation, or other causes, face higher risks from both chickenpox and shingles. They’re more likely to develop widespread rashes, internal organ involvement, and repeated reactivations.
Diagnosis
Chickenpox and shingles are often diagnosed by appearance alone, especially when the rash is textbook. But when the presentation is unusual, lab testing confirms the diagnosis. PCR testing, which detects viral DNA, is the most reliable method. The best samples come from scabs, blister fluid, or cells scraped from the base of a lesion. Older methods like viral culture and direct fluorescent antibody testing exist but are less sensitive. Blood tests for antibodies are a backup option when skin samples aren’t available, but they’re considerably less accurate than PCR.
Treatment Timing Matters
Antiviral medications work best when started early. For shingles, treatment should begin as soon as possible and ideally within one week of the rash appearing, or any time before the blisters have fully crusted over. For chickenpox, treatment should start as soon as lesions develop. A typical course of oral antivirals runs 7 to 10 days for shingles and 5 to 7 days for chickenpox. These medications don’t kill the virus, but they slow its replication, which reduces the severity and duration of symptoms and lowers the risk of complications like postherpetic neuralgia. Severe cases involving widespread skin lesions or internal organs require intravenous treatment in a hospital.
Vaccination
Two vaccines target varicella zoster at different stages of life. The chickenpox vaccine, given in childhood, prevents primary infection. The shingles vaccine (Shingrix) is a two-dose series recommended for adults 50 and older to prevent reactivation.
Shingrix is highly effective. In adults aged 50 to 69 with healthy immune systems, it prevents 97% of shingles cases. In adults 70 and older, effectiveness is 91%. Protection against postherpetic neuralgia specifically is 91% for those 50 and older and 89% for those 70 and older. These numbers make it one of the more effective vaccines available for older adults, especially given that one in three people would otherwise develop shingles in their lifetime.