Return of the Black Death: The World's Greatest Serial Killer
by Susan Scott and Christopher Duncan
318pp, John Wiley, £16.99

The Black Death 1346-1353: The Complete History
by Ole Benedictow
454pp, Boydell Press, £30

The medieval chronicler Gabriele de Mussis is often credited with the first description of biological warfare when he reported how in 1346, as the black death approached Europe, the Mongol armies besieging the Black Sea port of Kaffa catapulted plague-ridden corpses over the battlements. It is, however, like so much else surrounding the black death, a myth. The bodies of plague victims are not infectious, and neither the Mongol attackers nor the Christian defenders thought they were.

De Mussis, who was not present, was using the story as medieval tabloid journalism to illustrate Mongol frightfulness and corpse desecration. The "black death" is itself a misnomer. It is not because its victims turned black, they didn't, but is a later mistranslation of the contemporary term atra mors - meaning both terrible and black. And "Terrible Death" it certainly was.

When Alexander Yersin discovered the plague bacillis in 1894, named Yersinnia pestis after him, together with the meticulous work of the Indian Plague Commission, it was quickly concluded that this was the cause of bubonic plague, which in turn was the cause of the black death. Since then most experts have agreed with this conclusion. However, revisionist bacteriology is now almost as fashionable as revisionist history. In 2001 Norman Cantor suggested the black death was a form of anthrax, and now Susan Scott, a demographic historian, and Christopher Duncan, a zoologist, would have us believe the identification of the black death with bubonic plague is another myth.

Their case rests on an analysis of the spread of a plague epidemic in Penrith. In the autumn of 1597 an itinerant labourer called Andrew Hogson came to Penrith and lodged with the Railton family. He died suddenly on September 22. Three weeks later the two Railton children died of the plague, and mother and father followed soon after. By November 1597 the plague was in full swing, and the epidemic continued through the isolated valleys and communities for the next year. The black death had returned, as it had done ever since its first appearance in 1347, and would continue to do for another two centuries.

The crux of the argument rests on two claims. First, if Hogson brought the plague to Penrith and then infected his landlady and her children, the gap between his death and theirs, some three weeks, is too long for bubonic plague. If indeed he had directly infected the Railton family, that would be true. But he probably didn't.

Bubonic plague can spread directly from person to person via droplet infection. It causes pneumonic, rather than the more typical bubonic, plague, and is fatal within a day or so. If this was the only way the plague could spread, then the black death could not be due to bubonic plague since the victims would not have survived long enough to spread the plague beyond their immediate surroundings.

But the secret of the black death is that it has more than one way of transmitting itself. As every schoolboy knows, the villain was the black rat. Many rodents can harbour the plague bacillus, but the black rat is particularly susceptible. Plague has a devastating effect on black rat colonies, wiping them out. That would not matter, if it were not for the rat flea. A healthy black rat carries no more than half a dozen fleas. But a sick rat is unable to scratch them off, and before it perishes might be hosting several hundred. When the rat dies, the fleas look for their favourite host, another rat. So as unsuspecting humans go about their business, rats are dying like, well, flies. Only when the plague-carrying rat flea has run out of its preferred host does it move en masse to its second choice, humans. Now things move fast, and an explosive epidemic breaks out.

It will be self limiting. The sick humans die before they can move far, and the rats are already dead. But the damage has been done. While the epidemic was raging in the rats, but not the humans, people are on the move, and with them come rats and fleas. If they move by road, then the epidemic is transmitted slowly across the countryside. The black death travelled by this route at no more than a kilometre a day. But in the 14th century the best way to move people and goods was by sea and river. And it was along the shipping lanes and waterways that the plague was able to spread so devastatingly and bring Europe to a halt.

So the transmission of plague in Penrith is consistent with bubonic plague, provided that Penrith had its fair share of rats and fleas. But Scott and Duncan are convinced that it didn't. The black rat, unlike the shyer brown rat which replaced it across Europe in the 17th century, likes to live near humans and likes warmth. Scott and Duncan take this further, and assert that the black rat never inhabited the northern parts of Europe. As Britain and Scandinavia were indisputably devastated by the black death, it could not have been due to bubonic plague. The black rat, they say, is innocent.

Ole Benedictow, professor of history at Oslo University, is having none of this. His magisterial account mixes demographic research, meticulous reading of the chronicles and modern bacteriology to reaffirm that bubonic plague was indeed the cause of the black death. Archaeological evidence confirms the presence of the black rat across northern Europe even before the medieval period. The Scandinavians even had a word for it - the "Frankish mus" or French mouse. There was no shortage of black rats in northern medieval Europe. The rats were, however, not keen on the cold, and the historical record shows that only thing that could delay the relentless spread of the black death was winter, which did temporarily interrupt the cycle of bacillus, rat, flea and man.

Another myth surrounding the black death is that its spread was the consequence of crowded living conditions. But medieval Europe was not crowded. Only 2% of its population lived in communities larger than 10,000 people. Furthermore, the historical record shows that the plague was even more lethal in countryside than town, which on the surface runs counter to the normal rules of epidemics. But Benedictow shows how this too was the fault of the black rat. It is a territorial creature, and usually establishes itself as a single colony in a single house. People in medieval towns were less fastidious than their rats, and it was commonplace to find several families in a single dwelling, unlike in the countryside. So when the rat colony died, and released its plague-carrying fleas, they had more choice of victims in the town and any particular individual would have a slightly lower chance of receiving a fatal infection.

The black death also spread because nobody knew what to do about it. In 1348 Philip of France consulted the medical academics of the University of Paris who told him that the conjunction of Saturn, Jupiter and Mars was the reason for the calamity. Others believed the Jews were spreading the plague by poisoning the wells, ignoring the fact that Jews were dying just as quickly as their Christian neighbours. Murdering the local Jewish communities did nothing to stop the epidemic either.

The most popular explanation for the chaos engulfing society was that it was God's judgment on sinful mankind. The remedy was mass prayer or other demonstrations of hysterical piety, such as the Flagellants whipping themselves across the land. Still the plague spread.

Just occasionally the chronicles reveal glimpses of the strategy that eventually defeated the plague, and remains the basis of the management of epidemic infections to this day - quarantine, albeit crude and cruel. When the plague arrived in Milan, three households were infected. The authorities immediately bricked up the doors and windows, leaving the poor wretches, and their rats, to their fate. All perished, and Milan was spared. But Milan was the exception. The population of Europe in 1347 was about 80 million. Benedictow's meticulous analysis shows that this fell to 30 million in just six years, as he says, a "truly horrifying and even unnerving" statistic.

Europe had changed. Labour suddenly became scarce. Wages rose dramatically, signalling the end of the feudal system. The landowner saw rents and produce disappear, and was forced to look elsewhere for income. Benedictow argues that the prolongation of the hundred years' war was because the squire and knight needed plunder to replace the vanished rents, and that it took the black death to transform the Spanish hidalgos into conquistadors.

The world has moved on, but epidemics such as HIV and Sars continue to shape society. Could the black death happen again naturally? Or could al-Qaida unleash a new bubonic plague? The answer is no. The ecological niche of man, rat and flea no longer exists in most societies, while bubonic plague is now easily treatable.

But after a welcome debunking of some lurid biological terrorism scenarios, Scott and Duncan lob their own weapon of mass hysteria. If the black death was not bubonic plague, but an unknown viral infection possibly related to the lethal Ebola or Marburg viruses, then, as they recently told the BBC, it could be "merely lying in wait, ready to strike again", just as it did in 1347.

They paint an apocalyptic picture of a modern Britain returned to medieval conditions by such an epidemic, after which a new world order emerges dominated by a mysteriously plague-free Australia, a truly scary prospect. Luckily for us, their speculations have outrun their science. The black death was the bubonic plague. It changed the world once, but will not do so again.

· Simon Wessely is professor of psychiatry and director of the King's Centre for Military Health Research at King's College, London.