The last quarter of the 19th century had established beyond doubt the germ theory of infection over the miasmatic theory of disease. Over the last 50 years of the 19th century unparalleled advances had been made in medical science. Many of the causative organisms of what are now called tropical diseases had been identified. In 1851 Bilharz had discovered the worm which causes schistosomiasis or bilharzia; in 1876 Bancroft had isolated the filarial worm which causes the debilitating disease filariasis; the trypanosome had been discovered in 1877; in 1880 the French scientist Laveran had described the malaria parasite, Robert Koch the cholera vibrio in 1883, and in 1894 the plague bacillus was isolated. Crucial discoveries were made by Patrick Manson in 1879 in China on the transmission of filariasis by the mosquito, by David Bruce in 1896 on the transmission of bovine trypanosomiasis by the tsetse fly and by Ronald Ross in India in 1897 on the development of the malaria parasite in the mosquito.