One billion people in 43 countries are at risk of cholera -- a "pandemic killing the poor" -- despite prevention and treatment being relatively straightforward, the United Nations warned on Friday.

Between the World Health Organization and the children's agency UNICEF, the UN is seeking $640 million to fight the infectious disease, warning of a "cholera catastrophe" if action is not ramped up immediately.

"WHO estimates that a billion people across 43 countries are at risk of cholera," said Henry Gray, the UN health agency's incident manager for the global cholera response.