6. _Small-pox_ has an incubation period of ten to fourteen days. The disease begins with a chill, accompanied by violent headache. On the third day the eruption appears; little papules develop and quickly change into pustules, showing themselves first on the face, then on the back, arms, and hands, and finally on the legs and feet. On the ninth day the pustules suppurate, and after that gradually dry up; if it progresses favourably, the disease is over in two or two and a half weeks. In the case of small-pox the infective agent is not yet known; infection is caused by contact with a patient, or with objects which he is using or has used. It is particularly dangerous to touch things on which the contents of the pustules have dried, for such articles remain infectious for a long time. Recovery from the disease usually renders a person immune for life. The mortality in different epidemics varies greatly; most dangerous of all is the so-called ‘black small-pox’ (haemorrhagic small-pox, with bleeding in the pustules and under the skin). The total number of deaths in an epidemic of small-pox is dependent upon whether the disease appears in a vaccinated or an unvaccinated community; in the latter case the mortality may reach thirty per cent, whereas in the former case only three or four per cent of the patients die. Vaccination renders a person immune for eight to twelve years, while, if the disease breaks out anywhere in spite of vaccination, the number of fatal cases is very few. It should be noted that small-pox was formerly dreaded, not only because of its danger to life, but also because it frequently leaves a person disfigured for life, and in rare instances causes total blindness. 7. _Scorbutus_ (scurvy) used to be a common disease on ships, in prisons, and in times of famine; it appeared with the greatest malignancy in besieged cities—Thorn, Nuremberg, Alexandria, Port Arthur, &c. The real cause of the disease is unknown, although too much food of one kind, particularly lack of fresh vegetables, together with long confinement in poorly ventilated and dark rooms, are important causative factors. Inasmuch as the disease almost always appears in the form of an epidemic, it is probable that there is a specific infective agent. It begins with a general feeling of weakness; the skin and mucous membranes become pale and sallow, the gums become inflamed and ulcerated, and small and large extravasations of blood take place in the skin and muscles, and at the joints and knuckles. In serious cases haemorrhages occur in the intestines, kidneys, bladder, and uterus. A change of diet and surroundings will quickly cure scurvy; otherwise progressive anaemia will result in death. CHAPTER II THE TIME BEFORE THE THIRTY YEARS’ WAR