Nearly 20 percent of the world's population still defecates in the open, and action to improve hygiene, sanitation and water supply could prevent more than 2 million child deaths a year, health experts said.
In a series of studies on sanitation published as a cholera epidemic claims hundreds of live in Haiti, public health researchers from the United States and Europe found that this year 2.6 billion people across the world do not have access to even a basic toilet.
Unsafe sanitation and drinking water, together with poor hygiene, account for at least 7 percent of disease across the world, they said, as well as nearly 20 percent of all child deaths in the world.
Despite this, progress in improving safe water supplies and sanitation has been "painfully slow" in many developing countries, they said. They urged international donors, United Nations agencies, developing country governments and health workers to act now to reduce this "devastating disease burden".
Poor sewerage and sanitation can spread dangerous infections like viral hepatitis and cholera, an acute disease transmitted in contaminated water that causes watery diarrhoea and severe dehydration and can kill within hours if not treated.
More than 900 people have died of cholera in Haiti - which is still recovering from a devastating earthquake in January - in an outbreak which experts believe was worsened by flooding caused by Hurricane Tomas this month.
The United Nations forecasts up to 200,000 Haitians could contract the infection as the outbreak extends across the country and says $163.9 million in aid is needed over the next year to fight it.
In the studies, published in the Public Library of Science (PLoS) Medicine journal, researchers said that of the 2.6 billion people who have no access to decent sanitation, two-thirds live in Asia and sub-Sahara Africa.
It also found huge regional disparities in sanitation coverage. While 99 percent of people in industrialised countries have access to good sanitation, in developing countries only 53 percent have it. Within developing countries, urban sanitation coverage is 71 percent while in rural areas it is 39 percent.
"Globally, around 2.4 million deaths could be prevented annually if everyone practised appropriate hygiene and had good, reliable sanitation and drinking water," said Sandy Cairncross of the London School of Hygiene & Tropical Medicine, who led one of the studies. "These deaths are mostly of children in developing countries from diarrhoea and subsequent malnutrition, and from other diseases attributable to malnutrition."
A World Health Organisation (WHO) report published in May found the world was on track to achieve a globally agreed Millennium Development Goal (MDG) on access to safe drinking water, but more needed to be done to improve levels of sanitation.
The MDG targets call for the proportion of people without sustainable access to safe drinking water and basic sanitation to be halved by 2015 from levels in 2000.