From the beginning of European trade and conquest overseas, Europeans knew that strange “climates” could have fatal effects. Later, they came to understand that it was disease, not climate, that killed, but the fact remained that every trading voyage, every military expedition beyond Europe, had its price in European lives lost. For European soldiers in the tropics at the beginning of the nineteenth century, this added cost in deaths from disease – the “relocation cost” – meant a death rate at least twice that of soldiers who stayed home, and possibly much higher.

This book is a quantitative study of the relocation costs among European soldiers in the tropics between about 1815 and 1914, but it has broader implications. For Europe itself, this was the crucial century of the “mortality revolution,” with its profound influence on European and world demographic history. For the history of medicine, this was the transitional century between the kind of medicine that had been practiced in Europe since classical times and the kind of scientific medicine that would be spawned by the germ theory of disease. For Europe's global political and military relations, this marked the final period for the European conquest. For all these reasons, the relocation costs of this period have great bearing on human history.

In the longest run of time, human culture and human relationships to disease have passed through three phases. In the earliest, before the agricultural revolution of 10,000 b.c. – plus or minus a few millennia – human beings lived in small communities, as hunters, fishers, and gatherers.