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340 a village, or a district, to appear there later on. Particular houses, and even particular floors of houses, may be infected, whilst neighbouring ones remain free from the disease.

These and many other facts in the epidemiology of plague are probably to be explained by the connection of the disease with the rat - flea. When we have fuller information about the migrations of the rat, the breeding seasons of the rat-flea, the influence of food and temperature and other circumstances on these animals, it is reasonable to expect that our knowledge of the principles that underlie the transmission, the spread, and decline of plague will be more satisfactory than it is at present.

Symptoms. Incubation period.— Symptoms of plague begin to show themselves after an incubation period of from two to eight days. It is said that in certain very rare instances the incubation period may extend to as much as fifteen days. It is also said that in highly malignant epidemics the disease may declare itself within three or four hours from the time of exposure to infection. Prodromal stage.— In a certain but small proportion of cases there is a prodromal stage characterized by physical and mental depression, anorexia, aching of the limbs, feelings of chilliness, giddiness, palpitations, and sometimes dull pains in the groin at the seat of the future bubo. Stage of invasion.— Usually, the disease sets in somewhat suddenly with fever, extreme lassitude, frontal or, more rarely, occipital headache, aching of the limbs, vertigo, drowsiness or perhaps wakefulness, or troubled dreams. Rigor is rarely a marked feature; ore often the disease is heralded by feelings of chilliness. The face quickly acquires a peculiar expression, the features being drawn and haggard, the eyes bloodshot, sunken and staring, the pupils probably dilated sometimes the face wears an expression of fear or horror. The patient, when he can walk, drags himself about in a dreamy sort of way, or he staggers like a drunken man. There may