Unsafe water leads to one in 10 diseases and six percent of deaths worldwide, according to estimates in a report to be published by the World Health Organization on Thursday.
While the UN agency estimates that problems relating to water cause 9.1 percent of diseases recorded yearly, the report highlights this disproportionately affects the world's poorest countries.
"In the 35 most affected countries, over 15 percent of diseases could easily be prevented by improved water, sanitation and hygiene," report author Annette Pruss-Ustun told the press.
In developed countries, water causes less than one percent of deaths. In developing countries, the toll leaps to an average eight percent -- with the extreme case being Angola, at 24 percent.
Illnesses like malaria, dengue fever, or diarrhoea can be transmitted through the water supply.
The problem particularly threatens children. An unsafe water supply is to blame for 22 percent of illnesses in those under 14 years of age, and one quarter of young deaths.
As a solution, Pruss-Ustun encouraged governments to invest in this area. Every dollar invested would bring an additional benefit of eight dollars in healthcare savings and productivity, she said.