It must be remembered that septicæmic, bubonic and pneumonic plague are all manifestations of systemic infection with _B. pestis_; that they are all expressions of the same disease; that they call for the same treatment and that when the distinctive signs of bubo or pneumonia appear the disease is dangerously advanced. It should also be realized that every case is, almost from its onset, a septicæmic case, either mildly or overwhelmingly so. Accordingly the treatment should invariably be the treatment of septicæmic plague. The attitude of the diagnostician should be one of suspicion and he should have the courage to carry out antiplague treatment, practically upon suspicion. In this way only can the mortality of plague be greatly reduced. It is true of plague, just as it is true of cholera, that many of the fatal cases develop and become hopeless before the disease is suspected or diagnosticated. It is also true that many fatal cases of plague, in times of epidemic, completely escape recognition during life, the diagnosis being made in the autopsy room. Therefore, I lay great stress upon the necessity for an attitude of suspicion on the part of practitioners, wherever even a single case of plague (human or rodent) is known to have occurred. When it becomes necessary to establish a presumptive diagnosis, _i.e._, without resort to the microscope, the following symptoms and physical signs will be found to be most significant. SYMPTOMATOLOGY.--Acuteness of onset; rapidity of fever development; rapidity of the development of mental dulness or cloudiness, impairment of speech, delirium, stupor or restlessness; early and extreme prostration (perhaps more pronounced than in any other acute disease); extreme tenderness over involved gland masses, in the bubonic type of plague; cough, with considerable frothy sputum, soon becoming blood-discolored, in the pneumonic type of plague; and early cardiac asthenia in all clinical types of plague, septicæmic, pneumonic and bubonic. The following diseases may be confounded with plague, if symptoms alone are considered: typhus (_exanthematicus_), influenza pneumonia, broncho-pneumonia, severe malaria, septicæmia, acute toxic typhoid, venereal bubo, mumps and tonsillitis. I call attention again to the fact that mild cases of plague, septicæmic and bubonic, occur at times, clinical pictures in such cases being incomplete. The statement that the prognosis in all cases of septicæmic plague is hopeless is not confirmed by my experience. It should also be remembered that primary pneumonic plague and secondary pneumonia developing in the course of systemic plague are quite different in their significance and mortality, primary pneumonic plague being well nigh invariably fatal.