Chickenpox is caused by the varicella-zoster virus (VZV), a DNA virus in the herpesvirus family. The virus spreads easily between people through the air and through direct contact, making it one of the most contagious infections humans encounter. Before vaccination became routine in 1995, the United States saw more than 4 million cases every year.
The Virus Behind Chickenpox
VZV belongs to the same viral family as the viruses that cause cold sores and mono. Once it enters your body, it targets skin cells and triggers the familiar itchy, blister-like rash that defines the illness. Unlike many viruses that your immune system clears permanently, VZV never fully leaves. After the initial infection resolves, the virus retreats into nerve cells near the spine and skull, where it can stay dormant for decades.
How the Virus Spreads
VZV moves between people in three main ways: breathing in tiny airborne particles from an infected person’s skin blisters, inhaling potentially infectious respiratory droplets, and directly touching the fluid inside the blisters. The airborne route is what makes chickenpox so contagious. Viral particles from broken blisters can hang suspended in the air, meaning you don’t need to touch an infected person to catch it. Simply sharing a room or airspace with someone who has active chickenpox or a widespread shingles rash can be enough.
A person with chickenpox is contagious before they even know they’re sick, starting a day or two before the rash appears and continuing until every blister has crusted over. This pre-rash contagious window is a big reason the virus spreads so effectively through households, schools, and workplaces.
What Happens Inside Your Body
After VZV enters through your respiratory tract or eyes, it begins replicating in nearby lymph nodes. The virus then enters the bloodstream and spreads throughout the body. When it reaches the skin, it infects cells and causes them to fill with fluid, forming the characteristic blisters. New crops of blisters keep appearing for several days as the virus continues circulating, which is why you’ll often see spots at different stages (fresh red bumps, fluid-filled blisters, and dried crusts) all at the same time.
Most healthy children develop 200 to 500 blisters over the course of the illness, along with fever, fatigue, and loss of appetite. The whole process from first symptoms to the last crusted blister typically takes about a week to ten days.
Why the Virus Stays in Your Body
Once chickenpox clears, VZV doesn’t disappear. It travels along nerve fibers and settles into clusters of nerve cells called sensory ganglia, located along the spine and at the base of the skull. There, it enters a dormant state called latency, essentially hiding from the immune system indefinitely.
In some people, the virus reactivates years or decades later, causing shingles. When this happens, newly assembled virus particles travel back down nerve fibers to the skin, producing a painful, blistering rash that follows the path of a single nerve. The reactivation typically starts in one nerve cell, then spreads to surrounding support cells, which amplify the infection and funnel more virus toward the skin. This is why shingles appears in a band or strip on one side of the body rather than all over like chickenpox.
What triggers reactivation isn’t always clear, but a weakened immune system is the most consistent factor. Aging, stress, illness, and medications that suppress immune function all increase the risk.
Who Faces the Greatest Risk
Chickenpox is usually mild in healthy children, but certain groups are far more vulnerable to serious illness. People with weakened immune systems, including those undergoing chemotherapy, taking high-dose steroids, or living with conditions like leukemia or lymphoma, face the highest risk of severe disease. Children with HIV may develop an unusual form of the rash with new crops of blisters continuing for weeks or even months.
Pregnant women who catch chickenpox face a real danger of pneumonia, and the risk appears to be higher during the third trimester. If infection occurs during the first or early second trimester, the baby has a small chance (0.4 to 2%) of being born with congenital varicella syndrome, which can cause scarring, limb abnormalities, and neurological problems. Newborns are also vulnerable if the mother develops the rash within five days before or two days after delivery.
Healthy adults and teenagers older than 12 actually tend to get sicker from chickenpox than young children do. People with chronic skin or lung conditions and premature infants are also at elevated risk for complications.
Possible Complications
Most healthy people recover from chickenpox without lasting problems, but complications do occur. The most common in children is bacterial skin infection, where bacteria (particularly Group A strep) enter through broken or scratched blisters. More serious complications include pneumonia, brain inflammation (encephalitis), bloodstream infections, bleeding problems, and dehydration. These are uncommon in otherwise healthy people but become significantly more likely in the high-risk groups described above.
How Vaccination Changed the Picture
The introduction of the chickenpox vaccine in 1995 transformed the landscape of this disease in the United States. Two doses of the vaccine are about 90% effective at preventing chickenpox entirely, and those who do get a breakthrough case typically experience a much milder illness with fewer blisters and less fever.
The numbers tell a striking story. In the early 1990s, the U.S. recorded more than 4 million cases, 10,500 to 13,500 hospitalizations, and 100 to 150 deaths from chickenpox each year. Today, annual cases have dropped by more than 97%, to fewer than 150,000 cases, under 1,400 hospitalizations, and fewer than 30 deaths. Over its first 25 years, the vaccination program prevented an estimated 91 million cases and saved $23.4 billion in healthcare costs.
Children in the U.S. now receive their first dose around age 12 to 15 months and a second dose between ages 4 and 6. Adults and older children who never had chickenpox or the vaccine can still be vaccinated. Because VZV is so contagious, vaccination remains the most reliable way to prevent initial infection and, by extension, the later risk of shingles.