Chickenpox is a viral infection which spreads by droplets from the upper respiratory tract or by contamination from the discharge from ruptured lesions of the skin or through contact with a patient with herpes zoster. Herpes zoster appears to be due to reactivation of a latent infection with chickenpoxvirus. It may be accompanied by a varicelliform rash. Chickenpox is highly infectious and chiefly affects children under 10 years of age. Most children are little incommoded by this disease but, as often happens with viral infections, adults may develop a more severe illness including a prodromal rash. A suspicion of smallpox may easily be aroused. Second attacks are very rare. In patients on long-term steroid therapy the disease may be severe or even fatal. Most recorded fatalities have been suffering from leukaemia and it is probable that blood disorder has been the major factor in reducing resistance to the virus. The disease is not notifiable. The incubation period is about a fortnight. A quarantine period is not necessary.