The Black Death, generally thought to be bubonic plague, spread into Europe from the Caspian sea, brought by Mongols attacking the city of Kaffa in 1346. Carried by fleas on ship rats, it was ferried to ports throughout Europe. Over the next seven years it killed about a third of the population.
The transmission of an infectious disease in the Middle Ages is fairly predictable. While ships could transport the plague over long distances, its spread across the European continent from the Mediterranean ports resembles the steady advance of an inkblot. Human mobility was then very low: most people barely ventured a few miles beyond their hometown, and so infection advanced more or less village by village.
When human mobility was low, infectious diseases such as the Black Death spread slowly and predictably, like an inkblot (left). Today, humans criss-cross the globe in an instant (right), with potentially dire consequences for disease epidemics. (Credit (right): from B.Balcan et al., Proc. Natl. Acad. Sci. USA 106, 21484-21489 (2009).)
It’s different today. Individuals cross the globe in less than a day, while road, rail and water transport also allow rapid, long-distance movements. Increased human mobility recently has become a focus of attention because of fears about the spread of particularly virulent forms of influenza, such as H5N1 (bird flu) and H1N1(swine flu). Yet despite recognition of how much more mobile we are than in the fourteenth century, many models of epidemics still assume that diseases spread in smoothly advancing fronts. Largely this is due to a sheer lack of information about what patterns of human movement really look like.
Once these patterns are taken into account, the complexity of epidemiology is greatly increased: what seems at first like a purely medical question becomes linked to quite different areas of social science, such as the nature of transportation networks and their patterns of usage. At a local scale, modeling of epidemics might need to take account of the kinds of human movement models, which could determine for example how many people we encounter in our daily routines. Human movements, the nature and patterns of our social (and indeed sexual) intercourse, and variations in susceptibility to disease must also acknowledge the influence of socioeconomic demographics: affluence/poverty and culture affect all these things. In short, understanding the spread of disease today demands a truly cross-disciplinary perspective that, among other things, recognizes the complex interplay of many networks and modes of interaction.
The vital and urgent importance of that objective is in no doubt. AIDS is now the third biggest cause of premature death in the world, and kills two million people a year in sub-Saharan Africa alone. Around one in 20 people in that region aged between15 and 49 are HIV-positive, reaching a level of almost one in seven in southern Africa. The disease is partly responsible for a life expectancy of around 50and for preventing economic growth in the continent. Meanwhile, epidemiologists generally agree that a new flu pandemic, comparable to those in 1957–8 and1968–9 that killed millions worldwide, is inevitable and yet impossible to predict. Although combating such major health problems is partly a question of developing new drugs and understanding the biology of the pathogens, more than ever before it relies on understanding how the diseases are transmitted and spread through patterns of human movement and behavior. Only then, for example, can effective vaccination and quarantine strategies be devised.
Ball, Philip. Why Society is a Complex Matter: Meeting Twenty-First Century Challenges with a New Kind of Science.Springer-Verlag: Berlin, 2012