An early primitive form of the hazmat suit arose during bubonic plague epidemics, when European plague doctors of the 16th and 17th centuries wore distinctive costumes consisting of bird-like beak masks and large overcoats while treating victims of the bubonic plague At the time, it was thought that the inhalation of "bad air" was the cause of disease (a theory known as the miasma theory), so the bird-like beak masks functioned as respirators that contained aromatic items such as herbs and dried flowers. The modern hazmat suit is believed to originate from the Manchurian plague of 1910–1911, wherein Malayan physician Wu Lien-teh promoted the use of various forms of personal protective equipment to prevent the spread of the pneumonic plague