Today is World Toilet Day. You might chuckle or blush, but it’s worth taking a moment to acknowledge what the humble loo has done for us.

Though the word “toilet” is often considered declasse and even rude to utter aloud, much of modern life would not be possible without the commode. Ask yourself this: If you had to live without toilets or electricity, which would you choose?

If you opted for electricity, you might consider the plight of Londoners during the summer of 1858, when the city experienced what historians know as the Big Stink.

As a thriving metropolis at the peak of an empire, London teemed with vitality. But all those productive citizens had to poop, and all that excrement had to go somewhere.

Where it went, generally, was into chamber pots and thence into the streets or one of the city’s 200,000 backyard cesspits, which overflowed into basements, neighbors’ yards and nearby streets. Most of it ended up in the River Thames as undiluted, putrid muck. The problem was perennial, but the summer of 1858 was unusually hot, causing bacteria in the pits and river to multiply. The stench was so appalling the House of Commons was overpowered. Parliamentarians soaked the curtains in chloride of lime to combat the smell and considered moving their business upriver to Hampton Court. Anyone who could leave town did.

The experience galvanized the Metropolitan Board of Works, which set about reforming the city’s sanitation infrastructure. The next year, the major elements of the London sewerage system were under construction, which in turn necessitated the evolution of the flush toilet. Though the first modern toilet is said to have been built for Elizabeth I, true flushable loos are an invention of the late 19th century.

From the Middle Ages on, most large cities were drowning in excrement, making urban spaces not just stinky but downright dangerous. In the London cholera epidemic of 1844 to 1855, 20,000 people died because of commingling of sewage and drinking water. New York City began comprehensively building sewers in 1849, after its own series of deadly cholera outbreaks.

Toilets became a key factor in metropolitan growth both laterally and vertically. In order to build up, you have to be able to flush down. (Imagine carrying a chamber pot down the 102 stories of the Empire State Building.)

The average person goes to the toilet between six and eight times a day. Of the 69 gallons of water Americans use per person per day indoors, 27% (18.5 gallons) goes to flushing -- more than on showering or laundry or any other single activity, says the American Water Works Assn.

So central are flush toilets to our lives that we easily forget how many people do without them, or any other kind of effective sanitation either. Nobody seems to keep toilet statistics per se, but the World Health Organization and UNICEF monitor access to what is called “improved sanitation” -- which they reckon 2.5 billion people live without.

Contrary to what the term appears to imply, “it doesn’t mean anything indoors and it doesn’t mean water. It certainly doesn’t mean flushable toilets,” says Patricia Dandonoli, the president of WaterAID America, an organization that works to provide sanitation in developing countries. It does mean a private, covered pit latrine -- which Dandonoli stresses is “several steps up the sanitation ladder” from open defecation.

In cities like Nairobi, many residents have no option but to use what is known as the “flying toilet.” They defecate in a plastic bag that they throw onto the roadside. Others may only have access to the woods or open fields. In slums and shantytowns in South Asia and South America, millions of people rely on “hanging latrines,” an open platform with a hole in the middle built precariously on poles over a river or stream.

Women are especially vulnerable when there are no sanitation facilities. Modesty may keep them from going until night, when they are vulnerable to attack. In developing countries, 11% more girls go to school when sanitation is available.

Diseases such as diarrhea and dysentery -- caused by food and water contaminated with excrement -- are the second-biggest killer of children worldwide, causing 5,000 deaths a day, five times the number dying from HIV/AIDS. (The No. 1 killer is pneumonia and upper-respiratory infections.)

The costs of not having sanitation are enormous. According to the U.N. Development Program, countries in sub-Saharan Africa lose 5% of their total GDP because of illness and death caused by poor sanitation and water. The United Nations estimates that every $1 spent on sanitation saves at least $9 in accumulated health costs, lost productivity and delayed economic development.

One of the U.N.'s Millennium Development Goals, set in 2000, was to halve the proportion of people living without “basic sanitation” by 2015. We are nowhere near to making good on that promise, Dandonoli says. It would require spending about $10 billion a year for a decade to get covered pit latrines to just 50% of those who need them. That’s less than what Americans spend on bottled drinking water, which, according to the Beverage Marketing Assn., is running at more than $12 billion annually.

Today, on World Toilet Day, I propose that all of us give one loud grateful flush for the porcelain throne. Followed by one hour of not flushing in silent solidarity with those who don’t have the privilege of pooping safely in private.

Margaret Wertheim is a science writer who has consulted for WaterAID America on public outreach.