The city of Dhaka is surrounded by water, but it is so polluted that very little of it is drinkable. That, plus the very poor sanitation across the city, means that the people of the Bangladeshi capital not only have to scramble daily for clean water supplies but are also regularly in danger of the silent killer that is cholera. In Dhaka's Cholera Wars filmmakers Orlando de Guzman and Andrew Marshall visit the hospital that is bringing hope to the millions vulnerable to cholera in Bangladesh and elsewhere. Here they write about the experience.