The flea does not inoculate the bacillus by its bite. It sucks up blood containing plague bacilli. The latter multiply in the insect to such an extent that they block the entrance of the flea’s stomach and prevent it from feeding. The starved flea makes violent efforts to obtain more blood and, as a result, the contents of its gullet are discharged, together with the plague bacilli, upon the skin of the healthy person on whom it is trying to feed. If there is any little wound in this skin the bacilli gain an entrance and they set up the disease. Infection may also occur from the bacilli-containing excreta of the flea being voided on the skin and rubbed into wounds. When rats become ill or die the fleas leave them and attack man. It should be noted that the rat flea may remain infective for 43 days. Pneumonic plague, although due to the same bacillus, is quite a different kind of disease, and is transmitted from the sick to the sound by droplets of sputum expelled in coughing, and probably also by the invisible spray which pneumonia patients discharge from the mouth. When the disease is epidemic domestic animals may suffer from it and become sources of infection. _Symptoms._—Both forms of plague are characterised by sudden onset, sharp fever, giddiness, great weakness, a drunken gait, appearance and speech, and a tendency to heart failure. In bubonic plague there is lassitude, headache and shivering, and the face is pale and anxious. The patient looks haggard, his eyes are often bloodshot, and his expression is frequently one of fear or horror. His temperature runs up to 102°, 104° F., or even higher, and his face gets hot and flushed. There is intense thirst, the tongue becomes dry and brown, the urine is scanty, and there may be delirium. About the second or third day the glandular swellings known as buboes make their appearance, usually in the groin, but they may occur in the armpits, the neck, and elsewhere. As a rule there is only one bubo, which varies in size. There may be a good deal of pain associated with the swelling, and sometimes it is very severe. The bubo, if not dealt with surgically, eventually softens and bursts, discharging matter and sloughs. In cases which are going to recover, improvement is noted about the fourth or fifth day, and is heralded by a profuse perspiration. In fatal cases, death usually takes place between the third and fifth days. Pneumonic plague usually begins with shivering and vomiting. There is a cough, accompanied by breathlessness and blueness of the face, the sputum is profuse, watery and blood-stained, and as it is full of plague bacilli it is exceedingly dangerous. Very few cases of pneumonic plague recover.