Keo Samon, a rice farmer in southeastern Cambodia, had no toilet in her home. Nor was there even an outhouse or latrine for Keo and her husband and five daughters. Instead, they would defecate on land around their home, or in the rice fields.

That changed after the Water Supply and Sanitation Council, a United Nations partner, began to work with the village. Keo's family, along with 30 others, attended community-led awareness sessions, built simple dry toilets and joined the drive to make their village "open defecation-free".

"In the past, I did not know the consequences of defecating outdoors. It was simply my habit, like others in my village. We were not aware of the importance of good hygiene. But now, I am very excited to have my latrine," Keo said.

What good does a toilet do? More than you may imagine. Adequate sanitation prevents disease or malnutrition caused by contaminated water. Open defecation - practiced by more than a billion people around the world - is among the main causes of diarrhoea, which kills more than three quarters of a million children, aged 5 or under, each year.

Sanitation is also a necessary path to protection and empowerment for women and girls. When schools lack toilets, girls stay home when they are menstruating. When adequate sanitation is unavailable, women and girls are forced to take their private needs to the open, leaving them subject to sexual abuse.

Finally there is the economic argument. Poor water and sanitation costs developing countries around $260 billion a year - 1.5 percent of their gross domestic product. On the other hand, every dollar invested can bring a five-fold return by keeping people healthy and productive.

So, it is difficult to understand why, in 2013, 2.5 billion people around the world still lack access to adequate sanitation. More people have cellphones than toilets in today's world.

Since the adoption of the Millennium Development Goals in 2000, global poverty rates have been reduced by 50 percent, so has the number of people without access to improved sources of water, 200 million slum dwellers live better lives, and school enrolment has increased dramatically.

The global mobilization behind the MDGs has been a remarkable success that has changed the world's approach to development for the better. Yet, with just over 1,000 days remaining before the 2015 deadline for achieving the MDGs, we are not even close to reaching the goal on proper sanitation.

That is why I am launching a call to action on sanitation on behalf of the UN secretary-general, as we mark the beginning of the International Year on Water Cooperation.

There are three things we can do to speed up progress on sanitation. First, we should speed up the elimination of open defecation country by country, community by community, family by family. We need to talk about the problem, not turn our heads away from a subject many find uncomfortable.

Second, we need to strengthen cooperation. The water and sanitation challenge is everybody's business. We need everyone to play their part. National governments need to lead by making commitments. Local governments can work with communities to help them to help themselves. The private sector can invest in the health of their employees and the environment, and civil society organizations can monitor progress and advocate for solutions.

Third, we should scale up the projects that work. Simple, affordable actions have already proved their worth. Between 1990 and 2010, about 1.8 billion people gained access to sanitation - a significant achievement. Many countries have tackled this problem within a generation.

Doing nothing is not an option. The social, economic and environmental cost is simply too high. Let us commit now to ending open defecation and providing adequate sanitation and safe water for all, so women and girls can live with dignity, so our children can survive and communities thrive.

Keo in Cambodia reports that all her family members are now using the latrine. They are drinking safe water. "I ask all the families in my village to start building latrines for their use. This will help our village to end open defecation and bring good health for everyone, especially our children."

Keo has set an example. Let us follow one community at a time. Nobody can do everything, but everybody can do something.

The author is deputy secretary-general of the United Nations.