Causes and Risk Factors of Chickenpox

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chickenpox causes and risk factors
© Verywell, 2018

Chickenpox is a viral infection, which simply means that it's caused by a specific virus that spreads easily from person to person. Thanks to a highly effective vaccine for preventing chickenpox, the illness has become increasingly less common in the United States and other developed countries. People young and old do still get sick with chickenpox, however, and for some of them, the infection can lead to serious complications. That's why it's important to understand what causes chickenpox, who's most at risk of coming down with it, and how to protect yourself if you're exposed.

The Virus

The medical name of the virus that causes chickenpox is varicella zoster virus (sometimes referred to by doctors and researchers as VZV). Varicella is a herpes virus, putting it in the same family as the organisms that cause infections such as genital herpes and cold sores or fevers blisters.

VXV also is the virus that causes an extremely painful skin condition called shingles. Unlike other viruses, after a bout of chickenpox is over, the varicella virus hangs around in the nervous system rather than disappearing from the body. Shingles develops in older people who had chickenpox as kids when the virus is triggered to become active again.

Varicella is a humans-only virus, meaning you can't get chickenpox from a pet, or cause your dog or cat to become ill if you're sick. This is good to know since some infections that cause a rash, such as ringworm, can be transmitted between humans and animals.

Viruses such as varicella make people sick by invading healthy cells and using them to multiply, so when the body's immune system detects the presence of a virus in the body, it kicks into action, setting off symptoms that can be unpleasant but are designed to fight off infection.

Studies have found, for example, that a fever helps bolster the immune system. In fact, fever and other common symptoms of cold and flu often show up before the rash when someone comes down with chickenpox. This is particularly true of adults, according to the Centers for Disease Control and Prevention (CDC). So, although a specific virus is the cause of chickenpox infection, the symptoms are brought on by the unique way the immune system responds to the virus.

Risk Factors

Before the varicella shot became a regular part of the recommended vaccination schedule for children, chickenpox was most common in kids. And so arguably, the biggest risk factor for getting chickenpox was being a child under the age of 15. Now the risk factors for coming down with chickenpox boil down to:

  • Not being vaccinated: Coming in contact with VZV if you haven't been vaccinated isn't a guarantee you'll get sick, but the risk is high: The CDC says about 90 percent of unvaccinated people who come in contact with the virus will wind up with the illness. Getting the two-dose varicella vaccine is highly effective: According to the CDC, after the first shot, the vaccine is 85 percent effective at preventing varicella infection. After both doses, the vaccine is 98 percent effective at preventing varicella.
  • Never having had chickenpox: Once you've had chickenpox, your body will develop a lifelong immunity to it, so that even very direct contact with the varicella virus isn't likely to make you sick. But if you've never had chickenpox, you are at a high risk of getting sick if you're around others who have the illness. Again, the virus spreads incredibly easily, especially in close quarters. Unvaccinated kids are at increased risk of chickenpox if it's going around school or a daycare center, as are teachers and other adults who haven't been vaccinated or had the illness, for example.

    Special Concerns

    Most people who get chickenpox, especially children, are ill for a short period of time (about a week) and recover fully with no repercussions. But there are others who are at an increased risk of complications. They include:


    People who get chickenpox for the first time in adulthood are likely to have more severe symptoms and, according to the National Foundation for Infectious Diseases (NFID), "adults are more likely than children to die or have serious complications if they get chickenpox."

    People With Compromised Immune Systems

    This may include children who have leukemia or lymphoma; anyone with an immune-system disorder; and people who are taking a medication known to suppress the immune system, such as systemic steroids or chemotherapy drugs.

    Newborns Whose Mothers Become Infected With the Varicella Virus 

    Similarly, certain premature infants who are exposed to varicella or herpes zoster anywhere between five days before being born to two days after birth are at an increased risk of serious complications from the infection. Specifically, according to the CDC, these include:

    • Hospitalized preemies born at 28 weeks or later whose mothers aren't immune to the varicella virus
    • Hospitalized premature infants born at or before 28 weeks or who weigh 2.2 pounds or less at birth regardless of their mothers’ immunity status

    Pregnant Women With No History of Chickenpox or Vaccination

    The risk here is to their unborn babies. The NFID says that an infant born to a woman who gets chickenpox during the first 20 weeks of pregnancy has a one in 100 chance of developing serious birth defects ranging from shortened or scarred arms or legs; cataracts; small head size; abnormal brain development; and mental retardation.

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