Facts of the Plague . First traces of the plague was in 542 C.E.Is transmitted to humans by fleas found on ratsThree major forms of the disease are: Bubonic, Septicemic, and Pneumonic plague . Diseases caused by Yersinia Pestis . Bubonic plague: based on the lymphatic system.Septicemic plague: Centers in the bloodstream.Pneumonic Plague: Centers in the lungs. .