More than 1.5m children under five die each year because they lack access to safe water and proper sanitation, says the United Nations children's agency.
In a report, Unicef says that despite some successes, a billion people worldwide do not have access to safe drinking water from protected sources.
More than 1.2 billion people have gained access to safe water since 1990.
But sub-Saharan Africa remains a major area of concern, especially countries affected by conflict.
A Unicef deputy-director, Vanessa Tobin, gave the example of Niger, where only 13% of the population has access to toilets of an acceptable standard, or better.
She said it "certainly is a contributing factor in the cholera outbreaks" in Niger.
Progress slowed
The UN hopes to halve the number of people without access to clean drinking water and sanitation by 2015.
But progress has slowed due to population increases and unexpectedly high migration to urban areas, say the World Health Organisation and Unicef.
Ms Tobin said improving sanitation was key to helping prevent the spread of diseases such as cholera and diarrhoea-related illnesses.
"If we have clean water by itself without having sanitation and hygiene, we won't get the health impact."
The Unicef report says that children's education suffers because they have to walk long distances to fetch water, and that girls especially are deterred by the lack of separate and clean toilets in schools.
Diarrhoea-related diseases in young children could be cut by more than a third in young children by improving sanitation facilities, it adds.
The report picks out South Asia as a success story by prioritising sanitation. Access to improved sanitation facilities more than doubled in the region between 1990 and 2004.
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