 The credulous blamed emanations from the earth, "pestilential effluviums", unusual weather, sickness in livestock, abnormal behaviour of animals or an increase in the numbers of moles, frogs, mice or flies.It was not until 1894 that the identification by Alexandre Yersin of its causal agent "Yersinia pestis" was made and the transmission of the bacterium by rat fleas became known.The Great Plague in London had long been believed to be bubonic plague caused by "Yersinia pestis", and this was confirmed by DNA analysis in 2016.