Image copyright Science Photo Library Image caption The much-maligned black rat may not have caused repeated outbreaks of the plague 
 
 Black rats may not have been to blame for numerous outbreaks of the bubonic plague across Europe, a study suggests. 
 
 Scientists believe repeat epidemics of the Black Death, which arrived in Europe in the mid-14th Century, instead trace back to gerbils from Asia. 
 
 Prof Nils Christian Stenseth, from the University of Oslo, said: "If we're right, we'll have to rewrite that part of history." 
 
 The study is in the Proceedings of the National Academy of Sciences. 
 
 The Black Death, which originated in Asia, arrived in Europe in 1347 and caused one of the deadliest outbreaks in human history. 
 
 Over the next 400 years, epidemics broke out again and again, killing millions of people. 
 
 It had been thought that black rats were responsible for allowing the plague to establish in Europe, with new outbreaks occurring when fleas jumped from infected rodents to humans. 
 
 Rat reservoir 
 
 However, Prof Stenseth and his colleagues do not think a rat reservoir was to blame. 
 
 They compared tree-ring records from Europe with 7,711 historical plague outbreaks to see if the weather conditions would have been optimum for a rat-driven outbreak. 
 
 He said: "For this, you would need warm summers, with not too much precipitation. Dry but not too dry. 
 
 "And we have looked at the broad spectrum of climatic indices, and there is no relationship between the appearance of plague and the weather." 
 
 Image copyright Science Image caption When the weather is favourable, Asia's giant gerbils thrive, increasing the chance of plague transmission 
 
 Instead, the team believes that specific weather conditions in Asia may have caused another plague-carrying rodent - the giant gerbil - to thrive. 
 
 And this then later led to epidemics in Europe. 
 
 "We show that wherever there were good conditions for gerbils and fleas in central Asia, some years later the bacteria shows up in harbour cities in Europe and then spreads across the continent," Prof Stenseth said. 
 
 He said that a wet spring followed by a warm summer would cause gerbil numbers to boom. 
 
 "Such conditions are good for gerbils. It means a high gerbil population across huge areas and that is good for the plague," he added. 
 
 The fleas, which also do well in these conditions, would then jump to domestic animals or to humans. 
 
 And because this was a period when trade between the East and West was at a peak, the plague was most likely brought to Europe along the silk road, Prof Stenseth explained. 
 
 'Perfect storm' 
 
 "To me this was rather surprising," he said. 
 
 "Suddenly we could sort out a problem. Why did we have these waves of plagues in Europe? 
 
 "We originally thought it was due to rats and climatic changes in Europe, but now we know it goes back to Central Asia." 
 
 The team now plans to analyse plague bacteria DNA taken from ancient skeletons across Europe. 
 
 If the genetic material shows a large amount of variation, it would suggest the team's theory is correct. 
 
 Different waves of the plague coming from Asia would show more differences than a strain that emerged from a rat reservoir. 
 
 The plague died out in Europe after the 19th Century, however outbreaks continue to this day in other parts of the world. 
 
 The World Health Organization said there were nearly 800 cases reported worldwide in 2013, including 126 deaths. 
 
 In another paper, published in the American Journal of Tropical Medicine and Hygiene, researchers in the US said that the expansion of agriculture was placing East Africa at an increased risk of the plague. 
 
 As cropland increased, rodent populations were also rising, creating "the perfect storm for plague transmission", the researchers said. 
 
 Follow Rebecca on Twitter ||||| The 25 skeletons unearthed in the Clerkenwell area of London a year ago may hold the key to the truth about the nature of the Black Death that ravaged Britain and Europe in the mid-14th century. 
 
 A Channel 4 documentary on Sunday will claim that analysis of the bodies and of wills registered in London at the time has cast doubt on "facts" that every schoolchild has learned for decades: that the epidemic was caused by a highly contagious strain spread by the fleas on rats. 
 
 Evidence taken from the human remains found in Charterhouse Square, to the north of the City of London, during excavations carried out as part of the construction of the Crossrail train line, may support a theory held by some scientists that only an airborne infection could have spread so fast and killed so quickly. 
 
 The Black Death arrived in Britain from central Asia in the autumn of 1348 and by late spring the following year it had killed six out of every 10 people in London. Such a rate of destruction would kill five million now. By extracting the DNA of the disease bacterium, Yersinia pestis, from the largest teeth in some of the skulls retrieved from the square, the scientists were able to compare the strain of bubonic plague preserved there with that which was recently responsible for killing 60 people in Madagascar. To their surprise, the 14th-century strain, the cause of the most lethal catastrophe in recorded history, was no more virulent than today's disease. The DNA codes were an almost perfect match. 
 
 According to scientists working at Public Health England in Porton Down, for any plague to spread at such a pace it must have got into the lungs of victims who were malnourished and then been spread by coughs and sneezes. It was therefore a pneumonic plague rather than a bubonic plague. Infection was spread human to human, rather than by rat fleas that bit a sick person and then bit another victim. "As an explanation [rat fleas] for the Black Death in its own right, it simply isn't good enough. It cannot spread fast enough from one household to the next to cause the huge number of cases that we saw during the Black Death epidemics," said Dr Tim Brooks, a scientist from Porton Down, who was not part of the Crossrail team, will put forward his theory in a Channel 4 documentary, Secret History: The Return of the Black Death,on Sunday. 
 
 To support his argument, Brooks, who has yet to examine the Crossrail evidence, has looked at what happened in Suffolk in 1906 when plague killed a family and then spread to a neighbour who had come to help. The culprit there was pneumonic plague, which had settled in the lungs of the victims and was spread through infected breath. 
 
 The skeletons at Charterhouse Square reveal that the population of London was also in generally poor health when the disease struck. Crossrail's archaeology contractor, Don Walker, and Jelena Bekvalacs of the Museum of London found evidence of rickets, anaemia, bad teeth and childhood malnutrition. 
 
 In support of the case that this was a fast-acting, direct contagion, archaeologist Dr Barney Sloane found that in the medieval City of London all wills had to be registered at the Court of Hustings. These led him to believe that 60% of Londoners were wiped out. 
 
 Antibiotics can today prevent the disease from becoming pneumonic. In the spring of 1349, the death rate did not ease until Pentecost on 31 May. 
 
 • This article was amended on 3 April 2014 to make it clear that the pneumonic theory of the Black Death's spread is Dr Tim Brooks's own, as shown in the Channel 4 documentary. He is not associated with the Crossrail research. ||||| For centuries black rats have borne the brunt of the blame for the spread of the Black Death, which killed 25 million people across Europe during the first deadly pandemic in the mid-14th Century. 
 
 The plague was thought to have been largely transmitted by infected fleas living on rats, which would then bite humans. In total it is thought the disease has killed between 75 – 200 million people. 
 
 But has rattus rattus been unfairly accused of transferring the deadly bacteria? 
 
 Computer modelling carried out by a research team from the universities of Oslo and Ferrara suggests the first outbreak may not have been down to the rats, but instead can be “largely ascribed to human fleas and body lice”. 
 
 Using mortality data from nine outbreaks of the plague in Europe between the 14th and 19th centuries, the researchers plotted how the disease would likely spread by different means. 
 
 Seven out of the nine models indicated that transmission by human fleas and lice was the most likely cause of the disease’s spread, when compared to transfer by rats’ fleas, or airborne transmission. 
 
 In the paper, published in the Proceedings of the National Academy of Science, the researchers said: “While it is commonly assumed that rats and their fleas spread plague during the Second Pandemic, there is little historical and archaeological support for such a claim. 
 
 “Here, we show that human ectoparasites, like body lice and human fleas, might be more likely than rats to have caused the rapidly developing epidemics in pre-Industrial Europe.” 
 
 Speaking to the BBC, Profssor Nils Stenseth, from the University of Oslo said: “We have good mortality data from outbreaks in nine cities in Europe. 
 
 “So we could construct models of the disease dynamics [there].” 
 
 He added: “The conclusion was very clear. The lice model fits best.” 
 
 “It would be unlikely to spread as fast as it did if it was transmitted by rats. 
 
 “It would have to go through this extra loop of the rats, rather than being spread from person to person.” 
 
 Bubonic plague – believed to be the form of plague responsible for the Black Death – initially causes flu-like symptoms, and also causes lymph glands to swell up into painful “buboes”, often in the armpits, groin and neck. 
 
 People infected with plague require rapid treatment with antibiotics. 
 
 According to the World Health Organisation from 2010 to 2015 there were 3,248 cases reported worldwide, including 584 deaths. 
 
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 Currently, the three most endemic countries are the Democratic Republic of the Congo, Madagascar, and Peru.
– Much-maligned black rats and their fleas have long been blamed for spreading the Black Death that claimed millions of lives in Europe over the course of a few hundred years, but scientists now have a new critter they think was at fault. By studying tree rings to learn about precise climate fluctuations during the multiple epidemics from 1347 into the 1700s, a team out of the University of Oslo is reporting in the Proceedings of the National Academy of Sciences that giant gerbils in Asia were the more likely culprit. The working theory is that wet springs followed by warm summers in Asia prompted gerbil populations to soar, and that within a few years plague bacteria would show up in harbor cities in Europe, sweeping the continent from there. "If we're right, we'll have to rewrite that part of history," a researcher tells the BBC. "Suddenly we could sort out a problem," the researcher continues. "Why did we have these waves of plagues in Europe? We originally thought it was due to rats and climatic changes in Europe, but now we know it goes back to Central Asia." Next up: sequencing plague bacteria from skeletons across Europe. If there's considerable genetic variation in the disease, this will support the theory that new strains kept arriving from Asia, as opposed to persisting in rat reservoirs in Europe. Though not what it once was, hundreds of cases of the plague are still reported annually, with 126 deaths in 2013, reports the World Health Organization. (Check out what some researchers call the plague's silver lining.)