Corresponding to measles in its degree of contagiousness, chicken pox
occurs in extensive epidemics. In cities occasional cases appear at any
time, but at intervals epidemic outbreaks occur. Most children have
the disease during early years, but adults may also have it if they
have not come in contact with it in childhood. This disease is entirely
different from small pox and has no relationship to chickens. The cause
is unknown, but doubtless is a living germ. The crusts from the skin
have usually been blamed for the transferring of the disease from one
person to others. The disease however is contagious before the crusts
from the body have separated, and it is likely that the infectious
agent may be in the respiratory secretions early in the disease. One
attack protects for life. Second attacks are practically unknown.
The period of incubation which passes between the time of exposure and
the appearance of signs of the disease is quite long, being about three
weeks, varying in individuals between twelve and twenty-two days.