Varicella (chickenpox) is highly infectious (up to a 90% secondary attack rate of susceptible family members), and it spreads through inhalation of respiratory droplets. It has an incubation period of 14–16 days.
The typical varicella rash is first macular, becomes papular, and finally forms 200–500 blister-like vesicles that crust over. The rash or lesions begin on the scalp or face and spread downward, with a concentration of lesions on the trunk. Lesions appear in stages. Complications include infection of skin lesions; scarring; pneumonia; and, rarely, cerebral edema and death. Most varicella infections today are atypical, with fewer than 50–100 lesions, making clinical diagnosis more difficult and laboratory confirmation essential.
The risk of serious consequences of varicella increases with age; adolescents, adults, and immunocompromised persons typically have more severe cases of disease and are at higher risk for complications.
Visit CDC's Varicella information page for more information.