Agent Factors for Chicken Pox
Agent
The causative agent of Chicken Pox, V–Z virus is also called, “Human (Alpha) Herpes Virus 3”. Primary infection causes Chicken Pox. Recovery from primary infection is commonly followed by the establishment of latent infection in the sensory ganglia often for decades, without clinical manifestations. When the cell mediated immunity wanes with age or following immuno–suppressive therapy, the virus may reactivate, resulting in zoster, a painful, vesicular, pustular eruption in the distribution of one or more sensory nerve roots. The virus can be grown in tissue culture.

Source of Infection
Usually a case of Chicken Pox. The virus occurs in the oropharyngeal secretions and lesions of skin and mucosa. Rarely, the source of infection may be a patient with Herpes Zoster. The virus can be readily isolated from the vesicular fluid during the first three days of the illness. The scabs, however, are not infective.

Infectivity
The period of communicability of patients with varicella is estimated to range from one to two days before the appearance of rash, and four to five days thereafter. The virus tends to die before the pustular stage. The patient ceases to be infectious once the lesions have crusted.

Secondary Attack Rate
Chicken Pox is highly communicable. The secondary attack rate in household contacts approaches the 90% mark.

Host Factors for Chicken Pox
Age
Chicken Pox occurs primarily among children under 10 years of age. A few persons escape infection until adulthood. The disease can be severe in normal adults.

Immunity
One attack gives durable immunity; second attacks are rare. The acquisition of maternal antibodies protects the infant during the first few months of life. No age, however, is exempt in the absence of immunity. The lG antibodies persist for life and their presence is correlated with protection against Varicella. The cell mediated immunity appears to be important in recovery from V–Z infections and in protection against the reactivation of the latent V–Z virus.

Pregnancy
Infection during pregnancy presents a risk to the fetus and the neonate.

Environmental Factors
Chicken Pox shows a seasonal trend in India, the disease occurring mostly during the first six months of the year. Overcrowding favors its transmission. In temperate climates, there is little evidence of any seasonal trend.