Plague: The Mysterious Past and Terrifying Future of the World's Most Dangerous Disease

  • Wendy Orent
Free Press: 2004. 288 pp. $25

The threat of bioterrorism has put plague, a disease caused by the bacterium Yersinia pestis, into the spotlight. There are two main forms of the disease: bubonic plague develops following the bite from an infected flea and is characterized by the formation of swollen lymph nodes (buboes); pneumonic plague sometimes develops in patients suffering from bubonic plague, and the airborne transmission of the disease to other humans is then a very real possibility. Plague is one of the category A bioterrorism pathogens listed by the US Centers for Disease Control and Prevention in Atlanta, Georgia, so many researchers in the United States have recently redirected their efforts towards understanding the biology of this pathogen. But even before the threat of bioterrorism there was a fascinating story to be told. Books by Wendy Orent and by Susan Scott and Christopher Duncan provide the non-specialist with some insight into this story.

Taking its toll: the Black Death was a widespread killer in seventeenth-century London. Credit: THE ART ARCHIVE

It is generally accepted that there have been three great pandemics of plague. The first, known as the justinian plague, occurred in around AD 550 and was confined mainly to Africa and some parts of the Middle East. The second pandemic probably originated in central Asia and then spread along trading routes into Europe. It is this pandemic, which occurred mainly during the fourteenth and fifteenth centuries but rumbled on into the sixteenth and seventeenth centuries, that is referred to as the Black Death — it is believed to have killed up to 30% of the population of Europe. During the nineteenth century, after initial worldwide spread, the third pandemic of plague was confined mainly to Asia.

Both books do a good job of describing these pandemics. Orent describes all three, whereas Scott and Duncan focus on the Black Death in England — the outbreaks of disease in Penrith and Eyan are especially well researched and documented. Overlaid on these historical accounts are less convincing themes. In an attempt to describe the history of the Soviet bioweapons programme, Orent makes extensive use of interviews with US and Russian scientists. According to Russian former bioweapons scientists, plague was the favourite bacterial weapon of the Soviet military. Advanced genetic engineering techniques were reportedly used to create strains that were either resistant to antibiotics or that caused unusual forms of disease (although details of the make-up of these strains are not provided). US scientists interviewed by Orent are more sceptical of these claims.

Both books come to the same conclusion but from different angles — that Y. pestis, or at least the strains of Y. pestis that we are aware of in the West, could not have been responsible for the great pandemics of plague. Scott and Duncan argue that the symptoms and pattern of spread of the disease during the Black Death pandemic are inconsistent with our experience of plague during the twentieth century. They point out that fleas would be inactive during the winter months when cases of plague occurred and suggest that a virus may have been responsible for the Black Death.

Conversely, Orent points out that plague in England during the sixteenth and seventeenth centuries was seasonal, with most cases occurring during the summer months. Scott and Duncan also suggest that the incubation period of the disease was far in excess of that seen with plague. Part of the problem in interpreting the patterns of disease might lie with the assumption that it was either bubonic or pneumonic plague. For example, the authors correctly state that pneumonic plague is not sustainable for more than a few cycles of transmission. Less correctly, they suggest that pneumonic-plague patients would be too ill to travel and spread the disease. There are actually well documented twentieth-century examples of the disease spreading in this way.

Scott and Duncan also fail to consider some other possibilities. It is generally accepted that a complex interplay between the slowly spreading bubonic form of the disease and the explosive outbreaks of pneumonic plague occurred during the great pandemics, rather than one form alone. Significantly, Orent points out that this exact pattern of disease occurred during the Black Death. Scott and Duncan also fail to take account of the possibility that animal species other than rats played a role in the spread of disease and that fleas and flea bites were much more common between the fourteenth and seventeenth centuries than they are now.

Orent develops a different line of reasoning: that some strains of Y. pestis, and especially those found in marmots (large guinea-pig-like rodents) in Asia, are especially virulent. This is an interesting twist because the genome of Y. pestis is quite fluid and over past decades some researchers have suggested that hyper-virulent strains of Y. pestis might appear periodically.

Return of the Black Death: The World's Greatest Serial Killer

  • Susan Scott &
  • Christopher Duncan
Wiley: 2004. 256 pp. £16.99, $27.95

So what is the evidence that Y. pestis was responsible for any of the great pandemics of plague? Well, for the first two pandemics it is not strong enough to refute conclusively any of the arguments presented above. But the symptoms of plague during these great pandemics match the documented symptoms of today's plague. That Y. pestis was the aetiological agent of the third pandemic is irrefutable — the plague bacillus was first isolated and identified by Alexander Yersin during this outbreak. Recent molecular studies with Y. pestis provide additional evidence linking the bacterium with the earlier great pandemics. The ‘molecular clock’ (baseline mutations in housekeeping genes) suggests that Y. pestis evolved somewhere between 1,500 and 20,000 years ago, the former figure in remarkable agreement with the appearance of the justinian plague. Additionally, molecular phylogeny has revealed three genetically defined groups of Y. pestis, and these appear to correspond to the groups of strains associated with the three pandemics of plague.

It should be possible to obtain conclusive evidence that Y. pestis was the causative agent because a large number of corpses were buried anonymously under towns and cities during the Black Death. Viable bacteria will be long gone, but tell-tale DNA might still be present. But although one research group has reported the isolation of Y. pestis DNA from the teeth of presumed plague victims, others have been unable to repeat these studies.

Together these books give a good account of the history of the three great plagues. Other aspects are less convincing. We do not yet have conclusive proof that Y. pestis was the cause of the three great plagues. Although I am not yet convinced that the mysterious and unidentified virus suggested by Scott and Duncan was the cause of the Black Death, these books do serve to remind us never to be blinkered to other possibilities.