Thus far very little attention has been called to the fact that the age of the persons who succumbed to small-pox varied greatly in the different countries. This depends upon how well vaccinated the population of the country or countries was. Formerly, when nobody was ever vaccinated, the first year of life and the following years were by far the most seriously threatened; after the first few years the mortality of small-pox gradually decreased as the age of the patients increased. This also applies to-day to those countries in which vaccination is neglected. On the other hand, in those countries in which children are vaccinated in the first year of their lives, the infant mortality is low, although the same children lose their immunity to the disease when they grow older. To illuminate these facts let us adduce a few figures. In estimating the number of deaths, however, we cannot use the number of the living as a relative basis to work on, since the prevalence of small-pox varied greatly in the different countries; consequently we must take the total number of deaths and estimate the mortality on the basis of age from that alone. But in doing this we can compare with one another only entire countries in which the various ages are all about equally represented; if we were to take smaller units, for example, city and country, or agrarian and industrial districts, and use them for a basis of comparison, more detailed computations would be necessary. Of the four states included in the table below, Bavaria and Hesse introduced compulsory vaccination (the law required everybody to be vaccinated at least once) in the year 1807; Saxony and the Netherlands, on the other hand, did not have compulsory vaccination. Of every 100 persons who died of small-pox the following table indicates the relative proportion on the basis of age: _Vaccination not _Vaccination compulsory._ compulsory._ _Bavaria._ _Hesse._ _Saxony._ _Netherlands._ (1870–5). (1870–2). (1872). (1870–3). 0–20 years old 22·4 21·8 76·3 68·3 20–60 years old 59·0 65·4 21·9 29·6 Over 60 years old 18·6 12·8 1·8 2·1 This table clearly shows that vaccination protects a person against contracting small-pox for a number of years, or at least against succumbing to it, but that this immunity lasts only for a certain length of time and should be prolonged by revaccination—a fact which the Prussian military authorities recognized and took into practical consideration for many decades prior to the year 1870.