New York City Board of Health, 1832. The outdated public health advice demonstrates the lack of understanding of the disease and its actual causative factors. Seven<unk>  pandemics have occurred in the past 200 years, with the first pandemic originating in India in 1817. Additionally, there have been many documented<unk>  outbreaks, such as a 1991–1994 outbreak in South America and, more recently, the 2016–2021 Yemen<unk>  outbreak. Although much is known about the mechanisms behind the spread of<unk> , this has not led to a full understanding of what makes<unk>  outbreaks happen in some places and not others. Lack of treatment of human feces and lack of treatment of drinking water greatly facilitate its spread. Bodies of water have been found to serve as a reservoir, and seafood shipped long distances can spread the disease. Between 1816 and 1923, the first six<unk>  pandemics occurred consecutively and continuously over time. Increased commerce, migration, and pilgrimage are credited for its transmission. Late in this period (particularly 1879–1883), major scientific breakthroughs toward the treatment of<unk>  develop: the first immunization by Pasteur, the development of the first<unk>  vaccine, and identification of the bacterium Vibrio<unk> e by Filippo Pacini and Robert Koch. After a long hiatus, a seventh<unk>  pandemic spread in 1961. The pandemic subsided in the 1970s, but continued on a smaller scale. Outbreaks occur across the developing world to the current day. Epidemics occurred after wars, civil unrest, or natural disasters, when water and food supplies had become contaminated with Vibrio<unk> e, and also due to crowded living conditions and poor sanitation. Deaths in India between 1817 and 1860 in the first three pandemics of the nineteenth century, are estimated to have exceeded 15 million people. Another 23 million died between 1865 and 1917, during the next three pandemics.<unk>  deaths in the Russian Empire during a similar time period exceeded 2 million.
History of cholera
History of typhoid fever
History of smallpox
History of polio
History of malaria
History of chemical warfare
History of HIV/AIDS
History of plague