The story of David and Goliath comes in many versions. In William Rosen’s Justinian’s Flea, Goliath is the Roman empire, ruled in the 6th century AD by the emperor Justinian, from his magnificent capital in Constantinople. David is a deadly microscopic bug, Yersinia pestis, the cause of bubonic plague, which attacks its human victims on the back of the tiny flea. “The world’s smallest organism”, as Rosen puts it, versus “the world’s mightiest empire”.

In this version Goliath survived, but was badly hurt. Y-pestis caused the first known pandemic in history. According to Rosen’s generous estimates, 25m people died before it had run its course, perhaps 5,000 a day in Constantinople alone in AD542. It was a body blow to Roman imperial power that opened the way not just for the rise of Islam from the east, but also for the formation of the nation states of Europe in the west. Put at its strongest, Rosen’s claim is that the origin of modern geopolitics can be traced to some deadly flea bites in the mid-6th century.

If there is any period in the history of New Rome (or Byzantium as we now usually call it) likely to capture the modern imagination, it is the reign of Justinian. It has palace intrigue, murderous warfare, extravagant building schemes, and a literary tradition to rival the glory days of Old Rome, under Augustus or Nero. Justinian himself came from a one-horse town in the Balkans, but managed to reinvent himself in Constantinople and manoeuvre his uncle, then himself, onto the throne. He married an ex-prostitute by the name of Theodora, who is pictured respectably, along with her husband, in the mosaic of San Vitale in Ravenna. For much of his reign, his military campaigns were directed by the general Belisarius, a military strategist ranked with Alexander the Great and Napoleon, immortalised in Donizetti’s opera Belisario and in Robert Graves’s fictional biography Count Belisarius – a kind of Byzantine I, Claudius.

The exploits of these larger-than-life characters are recounted by a contemporary historian, Procopius, whose eight-volume history of the reign, The Wars, was consciously modelled on Thucydides. In fact, his description of Rosen’s 6th-century plague owes less to eyewitness observation than to Thucydides’s account of the (probably typhoid) epidemic at the beginning of the Peloponnesian war. But it is his scurrilous companion volume, The Secret History, that has given Justinian’s reign much of its colour. There, Procopius – now playing the misogynist moraliser – decries the behaviour of Theodora, her luxurious habits (such as a postbreakfast nap) and her sexual excesses. In one anecdote, he describes a theatrical star turn of her early career. She would “sprawl out” naked on the stage, while “some slaves sprinkled grains of barley over her private parts”. Then enter some geese, “who picked the grains off with their beaks, one by one, and ate them”. It was presumably, as Rosen notes, a saucy mythological mime – a “homage to the legend of Leda and the Swan”.

By and large, Rosen does a decent job with the reign of Justinian. There are a few troubling lapses: the learned bishop St Isidore would, I suspect, be horrified to be cast as a “physician”, just as Augustus would be puzzled to find his establishment of the Roman empire dated about 100 years too late. But overall, he conveys the significance and excitement of Justinian’s achievements, from the building of Hagia Sophia to the codification of Roman law, without falling for all Procopius’s lurid gossip.

He also does a good job in telling the scientific story of the plague. I am no more a professional microbiologist than Rosen is a professional historian (he is, in fact, a publisher), but I found his account of how the plague organism developed, how it mutated and was transmitted to humans, and how we can still track its progress on the ground, a fascinating one.

The problem is how the two halves of the book – the biological and the historical – fit together, and whether the impact of the plague on world history is as significant as he would like it to be, “midwiving” the birth of modern Europe as Justinian’s empire reeled. Of course, if the death toll was as large as he suggests, the plague must have made big waves in the political waters of the Mediterranean world. But I cannot help feeling that natural catastrophe is too slick an answer to be much use as a historical explanation on this scale.

As Rosen himself occasionally admits, between Justinian’s fleas and the drafting of the European constitution (his end point), the development of Europe is a rather more complicated process than a series of footnotes to a pandemic.

History is harder than this.

JUSTINIAN’S FLEA: Plague, Empire and the Birth of Europe by William Rosen

Cape £20 pp367

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