Varicella (chickenpox) is an acute, highly contagious disease caused by varicella-zoster virus (VZV). Following infection, the virus remains latent in neural ganglia and in about 10-20% of cases it is reactivated to cause herpes zoster, or shingles, generally in persons over 50 years of age or immunocompromised individuals. 

The infection may occasionally be complicated by pneumonia or encephalitis (inflammation of the brain), at times with serious or fatal consequences.

While mostly a mild disorder in childhood, varicella tends to be more severe in adults. It may be fatal, especially in neonates and in immunocompromised persons.

Varicella-zoster virus (VZV) transmission occurs via droplets, aerosols, or direct contact with respiratory secretions, and almost always produces clinical disease in susceptible individuals.