the sweating sickness, in the 14th century, was a horrible disease that killed thousands of people. it was caused by a bacterium called yersinia pestis and is believed to have been spread from bad flea bites. another theory believes that feverish conditions brought on by moldy bread made from wheat infected with ergot fungus could have been the cause of outbreaks in medieval times. one way or another, when people began dying at an alarming rate during epidemics, astute doctors knew what they were looking for – if there were any fleas found near them their symptoms would probably be cured upon killing the insect. then someone realized that blood-sucking insects themselves carried this bacterium and an effective cure could be established if sufferers stopped scratching at their sores to remove all traces of it!