The danger of the disease varies greatly in different epidemics; statements regarding this point diverge according as we refer to the statistical records of hospitals or to the private practice of physicians. With the latter the number of deaths is smaller, since persons suffering from the disease in mild form less often go to the hospitals. Epidemics in which a quarter of the patients, and even more, have succumbed have frequently occurred, especially in war times, during famines, &c. The cause (infective agent) of typhus fever is not known; according to recent investigations it is spread by vermin; Ricketts and others have fixed responsibility for it upon the body louse. The infection is communicated from man to man, and very often it is contracted from the clothes, linen, and other effects of typhus patients. Recovery from the disease usually renders a person immune against a second attack. Typhus fever frequently appears nowadays in the eastern and south-eastern parts of Europe, in Hungary and Galicia, and also in Spain, Italy, and Ireland. 2. _Plague_ appears in two forms, depending upon the place where the infective agent enters the body: the bubonic plague and the pneumonic plague. In the case of the former the painful plague-sores (buboes) develop, usually two or three days after infection, from the lymphatic glands; these sores,—which appear most often in the region of the groin, less often in the axilla, on the neck, lower jaw, and in other places,—soon suppurate. There is either a development of toxins, which are the cause of the severe general symptoms, or else the _bacilli pestis_ go from the glands into the circulatory system and cause septicaemia, which is quickly fatal. Pneumonic plague takes the form of a catarrhal inflammation of the lungs, causing a profuse and bloody expectoration, which contains large quantities of bacilli. This form of the disease almost always ends fatally in a few days. The mortality of bubonic plague is somewhat lower; the disease has an average duration of eight days, and carries away from fifty to seventy per cent of its victims. In the Middle Ages an epidemic of plague (black death) ravaged all Europe. At the present time it is still endemic in India, in southern China, in Egypt, in Uganda, and perhaps in other countries, whence it frequently develops into general epidemics.