Yes, varicella is caused by a virus. Specifically, the varicella-zoster virus (VZV) is a member of the herpesvirus family and the agent responsible for chickenpox. The word “varicella” is the medical name for chickenpox itself, while the virus behind it carries the full name varicella-zoster because it causes two distinct diseases: chickenpox on first infection and shingles if it reactivates later in life.
What Kind of Virus Is It?
VZV is classified as a human alphaherpesvirus, placing it in the same broad family as the viruses that cause cold sores and genital herpes. Like other herpesviruses, it contains double-stranded DNA wrapped in a protein shell and an outer envelope. This envelope is studded with proteins that help the virus latch onto human cells.
To infect a cell, VZV first attaches to sugar-based molecules on the cell surface, then binds to additional receptors that allow it to fuse with and enter the cell. Once inside, it hijacks the cell’s machinery to copy its own DNA and produce new viral particles. In the skin, the virus depends on the natural maturing process of skin cells to produce fully infectious particles, which is why the fluid-filled blisters of chickenpox are so contagious.
How Varicella Spreads
VZV is highly contagious and spreads through multiple routes. Direct contact with the fluid inside chickenpox blisters is one path. Inhaling tiny airborne droplets from those blisters is another. The virus can also travel through respiratory secretions that become aerosolized when an infected person coughs or sneezes. This combination of airborne and contact transmission is what makes chickenpox so efficient at spreading through households, classrooms, and other shared spaces.
What Happens During Chickenpox
After you’re exposed, the virus typically incubates for about 10 to 21 days before symptoms appear. The hallmark is an itchy rash that progresses through stages: flat red spots, raised bumps, fluid-filled blisters, and finally crusted scabs. New crops of blisters can continue appearing for several days, so you’ll often see lesions at different stages at the same time. Fever, fatigue, and loss of appetite are common alongside the rash.
You’re contagious starting one to two days before the rash shows up and remain so until every blister has crusted over, which generally takes about five to seven days after the rash begins.
Why the Virus Never Fully Leaves
One of the defining traits of all herpesviruses is latency, and VZV is no exception. After chickenpox resolves, the virus retreats into sensory nerve cells along the entire length of the spinal column and into nerve clusters near the brain. There it goes dormant, essentially hiding its DNA inside neurons without producing new virus.
The immune system keeps VZV in check for years or decades. But when immune surveillance weakens, whether from aging, stress, illness, or medications that suppress immunity, the virus can reactivate. It travels back along nerve fibers to the skin and produces a painful, blistering rash confined to a band or strip on one side of the body. This reactivation disease is shingles (herpes zoster). Research has shown that factors disrupting certain growth-signaling pathways in neurons can trigger reactivation, confirming that the balance between the virus and its host cell is an active, ongoing process rather than a simple on-off switch.
Serious Complications
For most healthy children, chickenpox is uncomfortable but self-limiting. The risk picture changes for certain groups. People with weakened immune systems, such as organ transplant recipients or those undergoing chemotherapy, face the danger of the virus spreading to internal organs. This can lead to viral pneumonia, liver inflammation, brain swelling (encephalitis), or a dangerous clotting disorder.
Pregnant women who contract chickenpox are at particular risk for pneumonia, which can become life-threatening. Some evidence suggests the risk is higher during the third trimester, though findings on timing aren’t entirely consistent. Infection early in pregnancy can also harm the developing fetus, causing a pattern of birth defects known as congenital varicella syndrome.
Vaccine Protection
The varicella vaccine uses a weakened live form of VZV to train the immune system. The standard schedule calls for two doses in childhood, and two doses are about 90% effective at preventing chickenpox altogether. When a vaccinated person does catch the virus, the illness is almost always milder, with fewer blisters and a lower risk of complications.
A separate, more potent vaccine exists for adults over 50 to prevent shingles. Because the chickenpox virus is already dormant inside anyone who had the disease, the shingles vaccine works by boosting immunity enough to keep VZV from reactivating.
Varicella vs. Other Viral Rashes
Several other viruses cause rashes that can look similar at first glance. Hand, foot, and mouth disease produces blisters but concentrates them on the palms, soles, and inside the mouth rather than spreading across the trunk. Measles causes a flat, blotchy rash that starts on the face and moves downward, without the fluid-filled blisters characteristic of chickenpox. Mpox produces deeper, firmer lesions that all tend to be at the same stage at once, unlike the overlapping crops of chickenpox blisters. The distinctive feature of varicella is that classic “dewdrop on a rose petal” appearance of its blisters, sitting on a red base, with lesions at multiple stages present simultaneously.