The Black Death was a devastating plague that killed many people in Europe in the mid-14th century. Medieval people originally thought it was caused by bad smells, but we now know it was transmitted via fleas that lived on rats traveling from China. The fleas would bite humans, infecting them with the plague bacterium Yersinia pestis. Common symptoms included swollen and painful lymph nodes (buboes), as well as fever, chills and coughing up blood or black sputum. Most cures attempted in medieval times were ineffective since the true bacterial cause was unknown.