Decades of chronic underfunding of water infrastructure is putting many countries at worse risk in the coronavirus crisis, with more than half the global population lacking access to safely managed sanitation, experts said as the UN marked World Water Day on Sunday.

Good hygiene – soap and water – are the first line of defence against coronavirus and a vast range of other diseases, yet three quarters of households in developing countries do not have access to somewhere to wash with soap and water, according to Tim Wainwright, chief executive of the charity WaterAid. A third of healthcare facilities in developing countries also lack access to clean water on site.

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“It’s really obvious that in Africa and parts of Asia we should be very fearful of what is to come,” he said. “The coronavirus crisis highlights how vulnerable the world is.”

The UN World Water Development report, published on Sunday, pointed to the underfunding of water infrastructure around the world, despite its importance.

Richard Connor, editor-in-chief of the report, told the Observer that water was often overlooked for spending and investment because the economic benefits of better water and sanitation were not emphasised. The coronavirus crisis sheds new light on those mistakes.

“One of the reasons underlying the investment gap in water and sanitation is that these services are perceived mainly as a social - and in some cases environmental - issue, rather than an economic one, like energy,” he said. “Yet the economic costs of an outbreak [such as Covid-19] are enormous, both in terms of national economies and stock markets, as well as in terms of household revenue - when people cannot work because of sickness or lockdowns. Realising the economic importance of water and sanitation should provide an additional catalyst for greater investment.”

Another reason for the neglect of water and sanitation is that people are generally willing to pay for the water coming into their homes, but not for transporting and treating afterwards. “Once it is flushed down the toilet, it disappears and becomes someone else’s problem,” said Connor. “Treating wastewater is several times more expensive than treating the source water in the first place. So without a willingness to pay on the part of users, it falls on governments to foot the bill, and since they do not recognise the economic value of wastewater treatment – which is perceived as more of an environmental issue - the political will behind such spending is low.”

Yet improving access to water and sanitation has clear benefits – in the coronavirus crisis, and beyond. Connor quotes evidence that suggests that the return on investment in water and sanitation can be high, with a global average benefit–cost ratio of 5.5 for improved sanitation and 2.0 for improved drinking water, when broader macroeconomic benefits are taken into account.

Water use has increased sixfold in the past century and is rising by about 1% a year owing to rising populations and increasing demand, while climate breakdown means that more areas of the world will see stress on their water supplies, including regions where supplies were previously abundant, such as many parts of Europe, Asia and north America.

One possible source for renewed investment in water is through a better understanding of the links between water issues and water infrastructure and the climate crisis, the UN report suggests.

While trillions in investment have been poured into reducing greenhouse gas emissions around the world in the last decade, through clean energy and low-carbon technology, few resources have been devoted to the water supply. This year’s UN water report has found that opportunities are being missed to use water projects to cut greenhouse gas emissions while improving access to clean water.

Sewage treatment is a clear example: wastewater gives rise to between 3% and 7% of all greenhouse gas emissions globally, more than flying. Processing sewage can turn wastewater from a source of carbon to a source of clean energy, if the methane is captured and used in place of natural gas. Currently, between 80% and 90% of wastewater around the world is discharged to the environment with no treatment.

Farming methods can also be adapted to use water more efficiently and cut carbon at the same time, because when soils are better managed they hold more organic matter, more carbon and more water – rendering them more fertile as well as sequestering greenhouse gases.

That makes investing in water a “win-win-win”, in terms of improving people’s lives, generating economic growth and helping to cut carbon, the report found.

Yet of the hundreds of billions in climate finance devoted to developing countries in recent years, projects involving water made up less than 1% in 2016, the latest year for which full figures were available, according to the report.

“Water does not need to be a problem – it can be part of the solution [to the climate crisis],” said Audrey Azoulay, director-general of Unesco. “Water can support efforts to both [reduce greenhouse gases] and adapt to climate change.”

Wainwright said the key ingredient for success in fixing the world’s water problems, alongside funding, was improving governance and how water supplies are managed.

“Water needs good governance,” he said. “That is usually what is missing. The world is not running out of water, but there is water stress. There is competition for water resources, but making sure that the people who need water get it is a good investment.”