Chickenpox is caused by the varicella-zoster virus (VZV), now classified as Varicellavirus humanalpha3. It is a double-stranded virus that belongs to the Varicellovirus genus and Orthoherpesviridae family. It is transmitted by the airborne route or direct contact with fluid from vesicles. VZV is highly contagious with 61–100% of household contacts with no prior infection developing an infection. Commonly known as a childhood infection, seroconversion in tropical countries occurs at a later age than in temperate countries, making chickenpox common in adolescents and adults in the former. VZV can also be transmitted to susceptible persons from patients with shingles. Studies suggest that the risk of viral transmission is considerably less from shingles than from chickenpox. The widespread vaccination of healthy children against chickenpox in the US has led to herd protection and extremely low levels of hospitalisation and mortality from chickenpox in children and adults.

Varicella may be more severe in pregnant women, particularly in the last trimester, with the potential for congenital varicella syndrome or foetal mortality. Maternal varicella that develops within 5 days before or 2 days after delivery is potentially most serious for the newborn resulting in congenital chickenpox infection. Maternal antibodies to VZV may not have been formed or crossed the placenta, and VZV may infect the baby before or after delivery. Vaccination of women before pregnancy is therefore recommended.