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How Chicken Pox Works

Chicken Pox Symptoms
The pesky pustules fill with fluid then scab over.
The pesky pustules fill with fluid then scab over.
Dr. Kenneth Greer/Getty Images

Once the replicated varicella virus gets into your bloodstream, your immune system recognizes the foreign invader and begins fighting it off after the incubation period. You feel that fight in the form of a fever, which lasts about two or three days. Fever often precedes adult cases of chicken pox, while children break out with the blistery rash first.

The blisters, or vesicles, are membranous sacs filled with clear liquid. They form on small red spots on the skin about one to four millimeters across [source: CDC]. The fluid inside the vesicles contains a chemical that stimulates the nerve cells at the skin. The nerves then alert the brain that the body itches. Although unpleasant, itching is actually a positive sign that the body is working to ward off the virus [source: Nemours Foundation]. Three or four days later, the itching typically subsides, and the blisters soon burst or darken and scab over.

The number and location of the vesicles can depend on age, skin condition and vaccination status. In general, the rash begins on the scalp, face and abdominal regions. From there, anywhere from 250 to 500 blisters will spread across the average unvaccinated person and may appear in the eyelids, mouth and genitals. When chicken pox strikes someone in spite of vaccination -- referred to as a breakthrough case -- it ordinarily results in 50 or so blisters.

After about a week, the blisters scab and dry out, and the virus is no longer contagious. Until then, patients should quarantine themselves at the first outbreak to avoid passing the virus to others. As the physical symptoms disappear, the virus deactivates but remains in the body in the nerve cells. Later in life, varicella may reactivate with stress or age as the herpes zoster virus, or shingles, which we'll discuss in the Shingles section.

This timeline of the varicella virus, from initial infection to inactivation, outlines what happens when you get chicken pox:

The stages of chicken pox, from virus to dormancy
The stages of chicken pox, from virus to dormancy
HowStuffWorks 2008

Rarely, a case of chicken pox develops into other, more serious illnesses and possibly death. In the next section, we'll discuss these complications and who should be concerned.