Severe disease occurs in immunocompromised and in neonates born to mothers who develop varicella 5 days before to days after delivery.
Humans are the only source of infection.
Spread occurs by direct contact, occasionally by air and rarely from zoster lesions. Nosocomial transmission is common on pediatric units and rare in newborn nurseries. Incubation is 10-21 days. Treatment with acyclovir in the first 24 hours reduces the severity and duration of disease. It is more useful in secondary household cases that are usually more severe.