With the outbreak of war in 1914, the decline was arrested and the movement of armies and migration of civil population to and from the endemic centres in the east of Europe rapidly made itself felt by an uncontrolled and increasing spread of smallpox which reached its greatest intensity in 1919. At the end of 1914 and throughout 1915 severe epidemic smallpox carried by prisoners of war from Russia and refugees occurred in the states of the old Austrian Empire which bordered on Russia and also in the Bal kans. Similar agencies scattered the infection throughout Ger many and in 1916-7 there were outbreaks in Hamburg and other western towns. The incidence of the disease in Germany rose con tinuously throughout the years of the War. Over 2,000 cases were recorded during 1917 and upwards of 5,000 during 1919. The infection reached Sweden on more than one occasion and a few cases occurred in Norway. In the Netherlands at least one serious outbreak occurred.
experience of France during the war period was remarkable. The serious epidemic which swept over the country during the war of 187o–I had not been forgotten and the French army of the World War was efficiently protected by vaccination. The invading German troops were also protected and no doubt acted as a screen to France on the east. Throughout the country, with the exception of districts on the Mediterranean coast and border ing on the Spanish frontier, the incidence amongst the civil popu lation was low during the War. The freedom of France was reflected in the freedom of England during this period. The British army, like the French, was vaccinated and therefore in contrast to the experience of England in 1870-71, and in spite of the enormous volume of cross-channel traffic, only a small amount of smallpox infection reached England through France. In the later years of the War the revolution in Russia sent many refugees into England who brought the infection via Scandi navia and the Murmansk coast. But the close guard maintained on the North Sea ports of Britain and the vigilance of her public health authorities prevented any serious attack on the civil population in spite of the increasing neglect of vaccination among the people.
In the third quinquennium, 1920-4, the bureau for the service of epidemiological intelligence and public health statistics was initiated under the authority of the League of Nations for the purpose of collection and interchange of information concerning epidemic diseases. The annual report from this source for 1924 deals with 29 European, 17 African, 20 American, 16 Asiatic countries and Australasia, and shows that in Europe smallpox incidence has diminished year by year since 1919. The effects of the War on the incidence of the disease in
Russia and neighbouring countries, viz.: Czechoslovakia, Ru mania, Austria, Poland, Finland and Germany reached their culminating point in 1919 and since that year the decline in prevalence in all these countries has been continuous and rapid. Similarly in Italy, where 34,365 cases were recorded in 1919, only 190 cases occurred in 1924. Although Spain continues to be the chief endemic centre in the west of Europe, the recorded deaths from smallpox in that country fell from 3,62o in 1919 to 1,214 in 1924. During the quinquennium England and Switzerland were the only European countries in which the incidence rose. In Switzerland the figure for 1924 shows a fall but in England the increase was progressive to the end of the period as seen by the following table In both countries the mild type of the disease, associated with the almost complete absence of fatality, led to the concealment of cases and an increasing neglect of vaccination.
Outbreaks of smallpox, like those of other infectious diseases, vary in their lethal power. In 1789 Jenner described an outbreak which occurred about seven years before in Gloucestershire, "of so mild a nature that a fatal instance was scarcely ever heard of . . . I watched its progress upward of a year without perceiving any variation in its general appearance. I consider it then as a variety of smallpox." Observers in different parts of the world have directed attention to the occurrence of outbreaks which apparently resemble the one described by Jenner. An outbreak in South Africa was de scribed by de Korte in 1904, under the name of Amaas or Kaffir pox, another by Ribas in Brazil in 1910, under the name of Alastrim, and others of a similarly mild character have been recorded in the United States, the West Indian Islands, Australia, and, as mentioned above, in England and Switzerland.
The opinions of many modern observers differ from that of Jenner who was satisfied in calling his outbreak a "variety of smallpox." That the form of disease observed in so many parts of the world belongs to the smallpox group is common ground. But some observers hold that it is more closely allied to chickenpox than to smallpox; others differentiate it from both diseases and regard it as a newly identified member of the group, while others maintain that it is a hybrid of smallpox and chickenpox, a true varioloid-varicella. On the other hand the weight of medical opinion in England inclines to the view that the disease is simply a mild type of smallpox.