 They are caused by "Yersinia pestis" bacteria spreading from flea bites through the bloodstream to the lymph nodes, where the bacteria replicate, causing the nodes to swell.Plague buboes may turn black and necrotic, rotting away the surrounding tissue, or they may rupture, discharging large amounts of pus.Infection can spread from buboes around the body, resulting in other forms of the disease such as pneumonic plague.