Chickenpox is one of the five classical childhood exanthems or rashes, once a cause of significant morbidity and mortality, but now chiefly of historical importance. Formerly one of the childhood infectious diseases caught by and survived by almost every child, its incidence had been reduced since the introduction and use of a varicella vaccine in 1995 in the U.S. and Canada to inoculate against the disease. Areas such as England, where the vaccine is not mandated, have increasing prevalence rates for chickenpox. Chickenpox is caused by the varicella-zoster virus (VZV), also known as human herpes virus 3 (HHV-3), one of the eight herpes viruses known to affect humans. It starts with conjunctival and catarrhal symptoms and then characteristic spots appearing in two or three waves, mainly on the body and head rather than the hands and becoming itchy raw pox (pocks), small open sores which heal mostly without scarring.Chickenpox has a 10-21 day incubation period and is highly contagious through physical contact two days before symptoms appear. Following primary infection there is usually lifelong protective immunity from further episodes of chickenpox.Chickenpox is rarely fatal (usually from varicella pneumonia), with pregnant women and those with a suppressed immune system being more at risk. Pregnant women not known to be immune and who come into contact with chickenpox may need urgent treatment as the virus can cause serious problems for the fetus. This is less of an issue after 20 weeks.The most common complication of chicken pox is shingles; this is most frequently a late effect.