The germ which causes measles has not been certainly isolated but it
is known to be in the secretions from the respiratory mucous membrane.
It is there in the earliest stages of the disease, two or three days
before the skin eruption appears, and it disappears when the eruption
fades. The danger of spreading the disease is therefore present very
early, before the eruption develops, and it is soon over, having
passed when fever has been absent a couple of days. The germs pass
from the sick to others in the secretions from the respiratory tract.
In coughing and sneezing small particles of infected mucus are thrown
out into the air as mouth spray and the inhalation of these causes
infection. Outside the body the germs quickly die. They do not survive
drying and exposure to the sunlight. Transfer of the disease by a
third person or by any mechanical carrier can only occur if it is done
quickly. The particles of moist secretion which convey the infection
may be very small and may be carried several feet in the air. Thus
a susceptible individual may be infected by coming into a room with
a case of measles although never approaching very close. Similarly
a child coming down with measles while in school may sow the germs
widely among other pupils.