Chickenpox is an infectious disease caused by the varicella zoster (DNA) virus, belonging to the herpes virus family (Alphaherpesvirinae) and related to herpes zoster. The virus initially presents as a chickenpox infection; however, because the virus is able to remain in the body's sensory nerve ganglia after the first infection, it has the potential to reactivate. If the virus reactivates, it presents as a shingles infection.⁽¹⁾ The virus is exclusive to humans, with outbreaks tending to occur between March and May⁽²⁾.

Transmission of chickenpox occurs through direct contact with the vesicles, inhalation of particles from the chickenpox vesicles, and possibly contact with respiratory secretions infected by the virus.⁽³⁾. Symptoms typically begin between 10 and 21 days after exposure and last approximately 5 to 10 days. In adults, initial symptoms include headache, fever, loss of appetite and tiredness, followed by the appearance of a rash. In children, the rash is often the first sign of infection⁽⁴⁾.

The rash begins with itchy red or pink papules that turn into fluid-filled blisters and generally last a few days before turning into fluid-filled blisters (vesicles). These vesicles break down after about a day and form crusts⁽⁵⁾. The rash usually spreads from the head to the rest of the body and can also appear on the eyes, throat, and genitals⁽⁶⁾. Since the rash appears over several days, chickenpox lesions may be present on the body in the form of papules, vesicles and crusts at the same time. On average, healthy children have between 200 and 500 chickenpox lesions, which typically range from 1 to 4 millimeters in diameter.⁽⁷⁾

 Chickenpox infections can occur in vaccinated people, but it is often a milder infection, with fewer than 50 skin lesions, and the rash often appears as papules rather than blisters. Fever rates may also be lower among vaccinated individuals who develop chickenpox. 25-30% of people who develop chickenpox after receiving a dose of chickenpox vaccine may still experience a disease similar to a natural chickenpox infection. Information about the presentation of chickenpox disease among people vaccinated with two doses of the vaccine is currently limited.8 A person with chickenpox chickenpox is still contagious and can spread the disease to others.⁽⁹⁾

A person with worsening chickenpox with fewer than 50 lesions is thought to be two-thirds less likely to transmit the disease than someone who has developed more than 50 lesions; however, the mild clinical presentation may delay the diagnosis. As a result, individuals with undiagnosed chickenpox have the potential to cause higher transmission rates within the community, by not taking isolation precautions to prevent spreading the disease to others.⁽¹⁰⁾

Chickenpox is considered a mild infection; however, complications can occur. Complications of chickenpox can include viral and bacterial pneumonia, bacterial skin infections, encephalitis, cerebellar ataxia, septicemia, necrotizing fasciitis, toxic shock syndrome, osteomyelitis, and septic arthritis.⁽¹¹⁾ Infants born to mothers infected with chickenpox between 5 days before and 2 days after delivery, premature infants, pregnant women, and people with immunosuppressed conditions are considered to be at higher risk of developing complications.⁽¹²⁾

Recovery from chickenpox confers long-lasting natural immunity, and immunocompetent individuals rarely suffer a second infection.⁽¹⁷⁾ Reexposure can boost immunity and decrease the risk of developing shingles.⁽¹⁸⁾