For years the Black Death in all its gory horror has been blamed on one animal - the rat.

But a new analysis of more than 100 plague epidemics which swept Europe for hundreds of years concludes that it was not spread by rodents running between villages and towns but by man himself, usually travellers unaware of their infection who moved between communities in search of work.

Chris Duncan, emeritus professor of zoology at Liverpool University, and social historian Sue Scott have carried out a piece of astonishing medical detective work, forming a complete history of the Black Death using parish records, wills and diaries across England.

They are convinced that the terrible illness was not in fact bubonic plague, which is carried by fleas on the backs of rats, but another highly infectious and deadly virus, haemorrhagic plague.

Their account of the Black Death shows it could easily have been transmitted by people travelling around the country, despite the lack of transport in the Middle Ages. It suggests the 20th century has badly misunderstood one of the worst epidemics ever to strike humanity.

Their book, The Return of the Black Death, is the result of years of work looking at how the plague spread so quickly, wiping out nearly one half of Europeans in medieval times.

It returned 300 years later, striking London in the middle of the 17th century, killing up to one-fifth of its population. The book ends by warning that other viruses may emerge equally quickly before we can do much to curb them.

Duncan and Scott became interested in what appeared to be inconsistencies in records. In Penrith, in Cumbria, they examined original parish registers dating back to 1538. 'One thing that really puzzled us was that we came across the death of a man called Andrew Hogson, who was marked down as a stranger in the records. His death was followed by a wave of plague deaths, yet there were no plague burials for another 22 days. It seemed a very long interval for a normal infection.'

Scott and Duncan went back to the original accounts of the plagues and began to chart how quickly people fell ill one after the other. They calculated that the infections would have had an incubation period of around 27 days.

This would have allowed enough time for travellers to move between villages and communities before they fell ill, and fits in with the information historians have about how the disease started in north Africa and spread up across Europe over the next three years.

Accounts from the 14th and 17th centuries describing plague symptoms say victims would suffer firstly from fainting, then sickness, followed by extremely painful swellings around the body. These symptoms fit more closely a description of haemorrhagic plague than the bubonic form.

By 1348 Europe was at war with this invisible enemy, but there was no cure and nothing approaching a public health plan to deal with it, other than for infected households to isolate themselves.

Many families who realised they were ill fled the towns, spreading the disease into the countryside.

There are several accounts of parents who abandoned their children once they realised they were ill. The tell-tale sign was the appearance on the skin of 'God's tokens' - the red rings which were a sign that the blood vessels were leaking blood into the tissue.

In The Decameron, Giovanni Boccaccio wrote of Florence: 'Many died by day or by night in the public streets; the departure of many others, who died at home, was hardly observed by their neighbours until the stench of their putrefying bodies carried the tidings; and what with their corpses and the corpses of others who died on every hand, the whole place was a sepulchre.'

Duncan is convinced, however, that the rat is completely innocent. He points out that there were no brown rats in Europe at the time and that the native black rats did not live in rural areas and would not have been resistant to the disease which would have been necessary for the infection to spread. He adds that in winter it would have been too cold for the fleas on rats to survive.

And what of the future? 'Haemorrhagic plague had a secret weapon which was hideously effective, which was its exceptionally long incubation period. We saw through the Sars epidemic that it is possible for viruses to escape far and wide because of global travel, unless the authorities are very alert to the dangers.

'If the Black Death happened now, you would try to isolate it as quickly as possible and then bring in a long quarantine period. But the same measures to each new case would have to be applied - the wearing of masks, the isolation, and the rigorous tracing of contacts.' Whether it would work is quite another question.

Eyewitness accounts

'Then a boil developed on their thighs... this infected the whole body, so that the patient violently vomited blood. This continued without intermission for three days, there being no means of curing it, and then the patient died'
· One of the first accounts of the Black Death - from Michael of Piazza, a Franciscan friar - in 1347

'August 31. In the city died this week 7,496, and of them, 6,102 of the plague. But it is feared that the true number of dead this week is nearer 10,000; partly from the poor that cannot be taken notice of, through the greatness of the number, and partly from the Quakers and others that will not have any bell rung for them'
· Samuel Pepys, in his diary, 1665