The disease

How does it differ from chickenpox?

In the past, smallpox was sometimes confused with chickenpox An acute contagious disease especially of children that is marked by low-grade fever and formation of vesicles and that is caused by a herpesvirus -- called also varicella. — a childhood infection that's seldom deadly. Yet chickenpox differs from smallpox in several important ways.

Chickenpox lesions An abnormal change in structure of an organ or part due to injury or disease; especially one that is circumscribed and well defined. are much more superficial and occur primarily on the trunk, rather than on the face, arms and hands. In addition, successive crops of lesions tend to develop in the same area. As a result, you'll often see a combination of scabs, vesicles A small abnormal elevation of the outer layer of skin enclosing a watery liquid and pustules A small circumscribed elevation of the skin containing pus and having an inflamed base. in someone with chickenpox.

A person infected with chickenpox can unknowingly transmit the virus to others before symptoms develop. Smallpox becomes infectious when symptoms appear and remains contagious until scabs fall from the pustules.

The World Health Organization (WHO Also known at the World Health Organization - WHO is the United Nations specialized agency for health that seeks the attainment by all peoples of the highest possible level of health. WHO is governed by 192 Member States through the World Health Assembly.) states that smallpox is most contagious after the fever begins and during the first week of the rash. You're less likely to become infected if you're exposed to someone in the latter stages of the disease.

Table 27-2 DIFFERENTIATION BETWEEN SMALLPOX AND CHICKENPOX
	Smallpox (Variola)	Chickenpox (Varicella)
Incubation Period The period of time between the infection of an individual by a disease-causing agent and the manifestation of the disease it causes.	7–17 days	14–21 days
Prodrome An early symptom of a disease.	Fever and malaise An indefinite feeling of debility or lack of health often indicative of or accompanying the onset of an illness. for 2–4 days before onset of rash	Minimal to none
Pock A pustule in an eruptive disease. Distribution	Centrifugal; usually on palms and soles	Centripetal; seldom on palms and soles
Pock Appearance	Vesicular A small abnormal elevation of the outer layer of skin enclosing a watery liquid—> pustular A small circumscribed elevation of the skin containing pus and having an inflamed base.—> umbilicated Having a small depression that resembles a navel.—>scab	Vesicular on erythematous base—>pustular—>scab
Evolution of Pocks	Synchronous	Asynchronous
Scab Formation	10–14 days after onset of rash	4–7 days after onset of rash
Scab Separation	14–28 days after onset of rash	Within 14 days after onset of rash
Infectivity	From onset of exanthem An eruptive disease (as measles) or its symptomatic eruption. until all scabs separate	From 1 days before rash until all vesicles scab

Chickenpox on the hand Notice the simultaneous occurrence of lesions in different stages of development: macules A patch of skin that is altered in color but usually not elevated and that is a characteristic feature of various diseases (as smallpox)., papules A small solid usually conical elevation of the skin caused by inflammation, accumulated secretion, or hypertrophy of tissue elements., vesicles A small abnormal elevation of the outer layer of skin enclosing a watery liquid, pustules A small circumscribed elevation of the skin containing pus and having an inflamed base., and crusts A hardened covering of dried secretions (as blood, plasma, or pus) that forms over a wound..

Smallpox close-up Notice that all lesions An abnormal change in structure of an organ or part due to injury or disease; especially one that is circumscribed and well defined. are in the same stage of development and that they are umbilicated Having a small depression that resembles a navel..