Cholera is an acute diarrhoeal infection caused by the bacterium Vibrio cholerae. Transmission occurs through direct or indirect oral contact with contaminated faeces or food and, in the most severe cases, may result in dangerous dehydration. During the 19th century, cholera often spread across the world from its original reservoir in the Ganges delta, causing six pandemics (a pandemic being an epidemic occurring over a very wide area, or globally) that killed millions of people worldwide.
The seventh pandemic is still ongoing: it started in South Asia in 1961, before reaching Africa in 1971 and the Americas in 1991. Cholera is now considered endemic in several countries, and the cholera-causing bacterium has not yet been removed from the environment.
Outbreaks can be caused by two serogroups of Vibrio cholerae: Vibrio cholerae 01 and Vibrio cholerae 0139. The main reservoirs of these pathogens are humans and water, especially brackish water in estuaries, often rich in algae and plankton.
Serogroup 01 is responsible for most of the outbreaks, and recent studies suggest that climate change may promote the creation of suitable environments for its spread. Serogroup 0139 was first identified in Bangladesh in 1992 and its spread has currently been confirmed only in South-East Asia. Other serogroups of Vibrio cholerae can be responsible for cases of mild diarrhoea that, however, do not develop into outbreaks.