The 7 billionth person on Earth will draw his or her first breath on Oct. 31, at least according to estimates by the United Nations. Assuming all systems are in working order, that baby will also create its first output that same day, in the form — to put it delicately — of a dirty diaper.

That dirty diaper is only the tip of an iceberg of human manure produced around the globe every day. It might seem a reasonable question to ask how humanity will deal with this output of feces as the world's population creeps toward 10 billion by 2100. But that question presumes we have the poop problem under control now. Here's the bad news: We don't.

Approximately 2.6 billion people around the world lack any sanitation whatsoever. More than 200 million tons of human waste goes untreated every year. In the developing world, 90 percent of sewage is discharged directly into lakes, rivers and oceans. And even in developed countries, cities depend on old, rickety sewage systems that are easily overwhelmed by a heavy rain.

All this untreated sewage adds up to a major public health crisis that kills an estimated 1.4 million children each year, according to the World Health Organization. That's one child every 20 seconds, or more than AIDS, malaria and tuberculosis combined. Despite this massive death toll, sanitation hasn't gotten the same attention as other world development goals. The United Nations, which set a goal to halve the number of people without basic sanitation by 2015, now calls that target "out of reach."

"Sanitation is not a sexy issue," said Dan Yeo, a senior policy analyst at WaterAid, a nongovernmental organization dedicated to hygiene, water and sanitation issues. "It's about s---, and that's not particularly attractive. It's a taboo to talk about in a lot of contexts." [See images of the world's toilets]

Learning to talk about toilets

That taboo is one reason that sanitation hasn't taken off as a major issue in the public's mind, Yeo said. But providing sanitation is also more complex than "if you build it, they will come," according to Rose George, the author of "The Big Necessity: The Unmentionable World of Human Waste and Why It Matters" (Metropolitan Books, 2008).

"The assumption was that latrines would be used and that everyone needs a flush toilet," George told LiveScience of early sanitation efforts. "People not necessarily wanting latrines was provedin India when the government provided millions almost free in the 1980s, and then millions of these kind-of-adequate latrines got turned into goat sheds or storage areas, because people are used to just going and crapping in the bush."

"Crapping in the bush," also known as "open defecation," is a major problem, George said, because the pathogens from the feces invariably end up tracked back into the village, often contaminating the community water supply.

Open defecation also puts people in rural areas such as sub-Saharan Africa at risk for snakebites as they go tramping into the bushes in the dark, George said. Women looking for a private place to go are at risk of being followed and sexually assaulted, she said. According to WaterAid, many women in Africa wait until nightfall to relieve themselves, putting themselves at risk of urinary tract infections, because propriety dictates that women don't go where someone might see them. [Sidebar: Top 16 Countries Without Sanitation]

To tackle the open-defecation problem, aid organizations had to learn to work with the people on the ground to explain why sanitation matters, Yeo told LiveScience. In Bangladesh, for example, WaterAid works with a local music-theater performance troupe that puts on sanitation-related skits for schoolchildren.

In her travels, George uncovered enormous cultural differences in the way people think about using the bathroom. In China, for example, plenty of public bathrooms lack doors on the stalls — or even stalls. Meanwhile, Americans happily use toilets in stalls with large gaps below, above and on either side of the door, a fact that seems bizarre in George's native Britain. In the U.K., she said, public toilet stalls are completely closed off.

"You have to understand that it's about software — psychology — as well as just the hardware of putting in pipes and toilets," George said.

Providing the plumbing

But the hardware matters, too. For one thing, the latrines can't be more disgusting than the alternative they're replacing, George said. Who wants to hang out in a dank, fly-infested latrine when you could just move your bowels down by the river?

Urbanization is another challenge, Yeo said. According to the U.N., the proportion of people living in urban slums around the world has declined from 39 percent in 2000 to 33 percent in 2010. But the absolute number of people living in slums is actually growing, with about 828 million slum-dwellers worldwide in 2010.

In many cases, these slums are informal communities that local governments would rather not recognize officially, Yeo said.

"They're often on land they don't own, and they aren't recognized as having the rights to that land," he said.

That makes officials reluctant to solve the sewage problems in these slums, since adding them to the grid would amount to tacit approval of their existence, Yeo said.

Meanwhile, just the physical layout of urban slums makes adding latrines difficult. A high density of human beings means a high density of human waste. Narrow streets make it tough to get latrine-emptying trucks into the area. In urban settlements, Yeo said, it's often important to encourage planning by local governments so these engineering problems don't blindside cities later on. [Read: How Many People Can Earth Support?]

Sewers and 'fatbergs'

Investing in sanitation is by any measure a winning bet: According to the U.N., for every dollar invested in sanitation, $8 are returned in reduced public health costs and lost productivity due to disease. According to WaterAid, a $30 donation buys one person access to both clean water and sanitation.

The availability of a toilet can have wide-ranging effects, George said. In developing areas, she said, up to 20 percent of girls drop out of school, because they have no place to relieve themselves. Providing a latrine can mean the difference between illiteracy and education.

But while the developing world undoubtedly bears the burden of poor sanitation, it would be a mistake to think that developed countries have it all figured out, George said. Urbanization and population growth have taken their toll on the crumbling sewer systems beneath many municipalities, she said, and many sewer systems are forced to release untreated sewage when a sudden downpour swamps the system.

"In the U.S., there's a massive, multimillion-dollar gap between the funding that is needed to maintain the sewer system and what is being given," George said. "Even a five-minute rainstorm can overwhelm the sewer system."

Even worse, she added, people's "out of sight, out of mind" attitude means they abuse the sewer system we do have.

"I've been down the London sewers, and all the 'flushers' who work down there say, 'We don't mind the s---, but we do mind the fat,'" she said.

The fat, George said, is household and restaurant grease that gets poured down drains and congeals into enormous "fatbergs," floating chunks of grease and oil. These grease bombs wreak havoc on an already strained system.

"We think we have it all sorted in the West," George said. "But we absolutely don't."

You can follow LiveScience senior writer Stephanie Pappas on Twitter @sipappas. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.