LONDON — In a world where more people have mobile phones than toilets, children are among the most vulnerable to contamination from human waste and dirty water.

At least 10 million children under age 5 have died since 2000 because they had no access to a basic toilet, according to a new report from the international development organization WaterAid.

About 2.5 billion, or 35 percent, of the planet’s 7 billion people have no basic sanitation facilities such as toilets and latrines, the U.N. says. An estimated 1.8 billion drink fecally contaminated water, according to World Health Organization/UNICEF figures.

In many poorer countries, inadequate hygiene and sanitation spread disease in households, schools and health centers. Poor conditions in communities and institutional settings, especially health facilities, exacerbated the spread of Ebola in West Africa, according to medical authorities.

Yet the water and sanitation picture isn’t all grim, said Jack Sim, founder of the World Toilet Organization. From 1990 to 2012, 2.3 billion people worldwide gained access to an improved drinking-water source, according to U.N.-Water Aid data.

In that time frame, child fatalities from diarrheal diseases – strongly associated with poor water, sanitation and hygiene – fell from about 1.5 million to about 578,000 children ages 4 and younger last year, a Lancet study showed.

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