About 650 million people, or one in 10 of the world's population, do not have access to safe water, putting them at risk of infectious diseases and premature death.
Dirty water and poor sanitation can cause severe diarrhoeal diseases in children, killing 900 under-fives a day across the world, according to United Nations estimates - or one child every two minutes.
Among newborn babies, the World Health Organisation says infections caused by a lack of safe water and an unclean environment cause one death every minute somewhere in the world.
The UN says access to safe drinking water and adequate sanitation services is vital to human health.
It is also important for other reasons - ranging from easily identifiable and quantifiable benefits such as cost and time savings, to more intangible factors like convenience, well-being, dignity, privacy and safety.
The WHO estimates that every dollar invested in improving water supply and sanitation services yields gains of up to $12, depending on the type of intervention.
This year's United Nations World Water Day is marked on March 22.
