Plague, rats, and ships The realisation of the infection routes of plague.

Three plague pandemics plus several epidemics have ravaged the world. The three pandemics were characterised by the role shipping played in spreading of the plague. The third pandemic, which began in southern China in the 1850s, was carried out of Hong Kong in 1894 to all continents by steamships. The oldest known documents mentioning quarantine as a precaution against epidemics dates back to 1127 in Venice. During the second pandemic, the Black Death, quarantine was systematised. During the third pandemic gassing of the ships was introduced by burning sulphur. Later hydrogen cyanide, carbon monoxide and other toxic gasses have been applied. In many harbours the use of rat shields were made compulsory in the beginning of the 20th century. The French bacteriologist Alexandre Emile Jean Yersin isolated in 1894 and identified Yersinia pestis as the contagious agent in Hong Kong despite obstructions from the British authorities who favoured Shibasaburo Kitasato from Japan. Four years later the French scientist Paul-Louis Simond established the rat flee, Xenopsylla cheopis, as the vector transferring the bacteria from rats to humans. This discovery was, however, not recognized until 1903 and another five years passed until clinical consequences were taken during the plague epidemic in India 1908. Each pandemic lasted several centuries due to reintroduction of Y pestis from local reservoirs in rodent populations in addition to reintroduction from the original Asiatic reservoirs.