ng
time and be continually discharged, as in the well-known case of
"typhoid Mary."[1]

Single cases of certain infectious diseases may appear in a community
year after year, and at intervals the cases become so numerous that
the disease is said to be epidemic. Such a disease is smallpox. This
is a highly infectious disease, towards which all mankind is
susceptible. Complete protection against the disease can be conferred
by Jenner's discovery of vaccination. The disease becomes modified
when transferred to cattle, producing what is known as cowpox, in
which vesicles similar to those of smallpox appear on the skin. The
inoculation of man with the contents of such a vesicle produces a mild
form of disease known as vaccinia, which protects the individual from
smallpox. This protection is fully as adequate as that produced by an
attack of smallpox, and we are warranted in saying that if thorough
vaccination, or the inoculation with vaccinia, were carried out
smallpox would disappear. There are great difficulties in the way of
carrying out effective vaccination of the whole population, which are
accentuated by the active opposition of people who are ignorant and
wilfully remain so. There exists in every state a number of people
unprotected by vaccination, and among these single cases of smallpox
appear. The unprotected individuals gradually increase in number,
forming an inflammable material awaiting the spark or infection which
produces a conflagration in the one case and an epidemic in the other.

Cerebro-spinal meningitis is another example of a disease which exists
in sporadic and epidemic form. This disease is caused by a small
micrococcus, the organisms joined in pairs. The seat of the disease is
in the meninges or membranes around the brain and spinal cord. The
micrococci enter the body from the throat and nose, and either pass
directly from here into the meninges, or they enter into the blood and
are carried by this into the meninges. The organisms are easily
destroyed and cannot long survive the conditions outside the body, so
that for infection to take place the transmission must be very direct.
Carriers who have the organisms in the throat, but who do not have the
disease, are the principal agents in dissemination. The mortality is
high, and even in recovery permanent damage is often done to the brain
or to the organs of special sense. Sporadic cases constantly occur in
small numbers, and it is difficult or imp