We have seen that insects serve as carriers of disease in two ways:  in one, by becoming contaminated with organisms they serve as passive carriers, and in the other they undergo infection and form a link in the disease.  The more recent investigations of modes of transmission of infectious diseases have shown that man, in addition to serving while sick as a source of infection, may serve as a passive carrier in two ways.  For infection to take place not only must the pathogenic organism be present, but it must be able to overcome the passive and active defences of the body and produce injury.  Pathogenic organisms may find conditions favorable for growth on the surfaces of the body, and may live there, but be unable to produce infection, and the individual who simply harbors the organisms can transmit them to others.  Such an individual may be a greater source of infection than one with the disease, because there is no suspicion of danger.  The organisms which thus grow on the surfaces have in some cases been shown to be of diminished virulence, but in others have full pathogenic power.  Such passive carriers of infection have been found for a number of diseases, as cerebro-spinal meningitis, diphtheria, poliomyelitis and cholera.  In all these cases the organisms are most frequently found in those individuals who have been exposed to infection as members of a family in which there have been cases of disease.  The other sort of carrier has had and overcome the disease, but mutual relations have been established with the organism which continues to live in the body cavity.  Diphtheria bacilli usually linger in the throat after convalescence is established, and until they have disappeared the individual is more dangerous than one actually sick with the disease.  Health officers have recognized this in continuing the quarantine against the disease until the organism disappears.  In typhoid fever bacilli may remain in the body for a long 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