In 1894 scientist discovered a bacterium known as Yersinia Pestis. It was believed to be known that there were three form of the plague. The first form was transmitted through bacteria by a number of different types of flees. The second form of the plague was also carried by flees and transmitted to rats who then carried the germ. The final form of the plague was the most deadliest known as the Pneumonic plague which was transmitted by humans. The pneumonic plague inflected the lungs therefore turning the plague to be the most contagious. It caused repeated coughing which allowed bacteria to contaminate the air resulting in humans being infected in just one breath. The plague was believed to have originated in Mongolia in the early 14th century and spread along trade routes via military expeditions. It was believed to have been dormant but in 1900 it arrived in Australia killing over 550.