Diphtheria is an acute infective disease due to the action of a specific bacterium, the _bacillus diphtheriæ_ or _Klebs-Löffler bacillus_. The disease is usually transmitted from one patient to another, but it may be contracted from cats, fowls, or through the milk of infected cows. Cases have occurred in which the surgeon has carried the infection from one patient to another through neglect of antiseptic precautions. The incubation period varies from two to seven days. #Clinical Features.#--In _pharyngeal diphtheria_, on the first or second day of the disease, redness and swelling of the mucous membrane of the pharynx, tonsils, and palate are well marked, and small, circular greenish or grey patches of false membrane, composed of necrosed epithelium, fibrin, leucocytes, and red blood corpuscles, begin to appear. These rapidly increase in area and thickness, till they coalesce and form a complete covering to the parts. In the pharynx the false membrane is less adherent to the surface than it is when the disease affects the air-passages. The diphtheritic process may spread from the pharynx to the nasal cavities, causing blocking of the nares, with a profuse ichorous discharge from the nostrils, and sometimes severe epistaxis. The infection may spread along the nasal duct to the conjunctiva. The middle ear also may become involved by spread along the auditory (Eustachian) tube. The lymph glands behind the angle of the jaw enlarge and become tender, and may suppurate from superadded infection. There is pain on swallowing, and often earache; and the patient speaks with a nasal accent. He becomes weak and anæmic, and loses his appetite. There is often albuminuria. Leucocytosis is usually well marked before the injection of antitoxin; after the injection there is usually a diminution in the number of leucocytes. The false membrane may separate and be cast off, after which the patient gradually recovers. Death may take place from gradual failure of the heart's action or from syncope during some slight exertion. _Laryngeal Diphtheria._--The disease may arise in the larynx, although, as a rule, it spreads thence from the pharynx. It first manifests itself by a short, dry, croupy cough, and hoarseness of the voice. The first difficulty in breathing usually takes place during the night, and once it begins, it rapidly gets worse. Inspiration becomes noisy, sometimes stridulous or metallic or sibilant, and there is marked indrawing of the epigastrium and lower intercostal spaces. The hoarseness becomes more marked, the cough more severe, and the patient restless. The difficulty of breathing occurs in paroxysms, which gradually increase in frequency and severity, until at length the patient becomes asphyxiated. The duration of the disease varies from a few hours to four or five days.