until the disappearance of the eruption. The disease
ordinarily occurs in epidemics, but occasionally in isolated cases.

=Development.=--A period of two weeks commonly elapses after exposure
to the disease before the appearance of the first symptom of
chickenpox, but this period may vary from thirteen to twenty-one days.

=Symptoms.=--The characteristic eruption is often the first warning of
chickenpox, but in some cases there may be a preliminary period of
discomfort, lasting for a few hours, before the appearance of the
rash; particularly in adults, in whom the premonitory symptoms may be
quite severe. Thus, there may be chilliness, nausea, and even
vomiting, rarely convulsions in infants, pain in the head and limbs,
and slight fever (99 deg. to 102 deg. F.) at this time. The eruption shows
first on the body, in most cases, especially the back. It consists of
small red pimples, which rapidly develop into pearly looking blisters
about as large as a pea to that of the finger nail, and are sometimes
surrounded by a red blush on the skin. These blisters vary in number,
from a dozen or so to two hundred. They do not run together, and in
three to four days dry up, become shriveled and puckered, and covered
with a dark-brown or blackish crust, and drop off, leaving only
temporary red spots in most cases. The fever usually continues during
the eruption. During the first few days successive fresh crops of
fresh pimples and blisters appear, so that while the first crop is
drying the next may be in full development. This forms one of its
distinguishing features when chickenpox is compared with smallpox. In
chickenpox the eruption is seen on the unexposed skin chiefly, but may
occur on the scalp and forehead, and even on the palms, soles,
forearms, and face. In many cases the eruption is found in the mouth,
on its roof, and the inside of the cheeks. The blisters rarely contain
"matter" or pus, as in smallpox, unless they are scratched. Scratching
may lead to the formation of ugly scars and should be prevented,
especially when the eruption is on the face. Pitting rarely occurs.

=Determination.=--The discrimination between chickenpox and smallpox
is sometimes extremely puzzling and demands the skill of an
experienced physician. When one is unavailable, the following points
may serve to distinguish the two disorders: smallpox usually begins
like a severe attack of _grippe_, with pain in the back and head,
general pains and naus