Shigeru Omi, World Health Organisation director for the Western Pacific, said 1.8 million worldwide die every year from diarrhoeal diseases mainly attributable to unsafe water supplies, inadequate sanitation and poor hygiene.
"The majority of these deaths occur in Asia, with 90 percent of them children under five years of age. In fact, diarrhoea is the second leading cause of childhood death in developing countries," he said.
He was speaking at the launch of the World Toilet Association, a South Korea-based non-governmental organisation aimed at helping the world's 2.6 billion people who live without toilets.
Two thirds of them live in Asia, with more than half in China and India alone, according to Omi.
Studies have shown that better sanitation can reduce diarrhoeal deaths by up to 32 percent and hygiene campaigns such as promoting hand-washing can reduce such deaths by up to 45 percent, he said.
"Just imagine the number of children whose lives could be saved through simple low-cost interventions in sanitation and hygiene," he said.
Many other illnesses are also related to unsafe water and poor sanitation and hygiene, such as intestinal worm infections.
Currently, about 133 million people worldwide suffer high-intensity worm infections which often lead to cognitive impairment, dysentery or anaemia, Omi said.
The UN has set a goal of helping at least half of the 2.6 billion people lacking proper toilets by 2015 at an estimated cost of 10 billion dollars worldwide per year.
The annual cost, Omi said, is less than one percent of world military spending in 2005. "It is one third of the estimated global spending on bottled water. Ten billion is about as much as Europeans spend on ice cream each year."
A recent report found that investment in water and sanitation in East Asia would return economic gains of some six dollars for every dollar invested, he said.
Providing everyone in East Asia with safe water and basic sanitation would also reduce health care costs by eight billion dollars per year, and would avert costs of 40 billion dollars in the form of time lost fetching water from afar.
The World Toilet Association was launched by South Korean campaigner Sim Jae-Duck, who takes the subject so seriously he has even built a 1.6 million dollar toilet-shaped house to promote his campaign.
Its inaugural meeting drew some 1,300 people from more than 60 countries including health officials and sanitation industry representatives.
A draft "Seoul Declaration" calls for international campaigns to provide proper sanitation, tackle water pollution and mount public education campaigns.
Ann M. Veneman, executive director of UNICEF, said in a video message that the impact of poor sanitation extends beyond health.
"Where schools do not provide proper toilets for children, particularly for girls, their educational prospects suffer.
"Clean, safe and dignified toilet and hand-washing facilities in schools help ensure that girls get the education they need and deserve," she said.
"When girls get an education, the whole community benefits."