Maria del Carmen Abadía lives in one of Mexico’s rainiest regions, but she has running water only once every two days. When it does trickle from her tap, the water is so heavily chlorinated, she says, it’s undrinkable.

Drinking water is increasingly scarce in San Cristóbal de las Casas, a picturesque mountain town in the southeastern state of Chiapas, where some neighbourhoods have running water just a few times a week, and many households are forced to buy extra from tanker trucks.

So many residents drink Coca-Cola, which is produced by a local bottling plant, can be easier to find than bottled water and is almost as cheap.

In a country that is among the world’s top consumers of sugary drinks, the region Chiapas is a champion: residents of San Cristóbal and the lush highlands that envelop the city drink on average more than 2 litres, or more than half a gallon, of soda a day.

The effect on public health has been devastating. The mortality rate from diabetes in Chiapas increased 30 per cent between 2013 and 2016, and the disease is now the second-leading cause of death in the state after heart disease, claiming more than 3,000 lives every year.

“Soft drinks have always been more available than water,” says Abadía, 35, a security guard who, like her parents, has struggled with obesity and diabetes.

Vicente Vaqueiros, 33, a doctor at the clinic in San Juan Chamula, a nearby farming town, says healthcare workers are struggling to deal with the surge in diabetes.

“When I was a kid and used to come here, Chamula was isolated and didn’t have access to processed food,” he says. “Now, you see the kids drinking Coke and not water. Right now, diabetes is hitting the adults, but it’s going to be the kids next. It’s going to overwhelm us.”

Buffeted by the dual crises of the diabetes epidemic and the chronic water shortage, residents of San Cristóbal have identified what they believe is the singular culprit: the hulking Coca-Cola factory on the edge of town.

The plant has permits to extract more than 300,000 gallons of water a day, as part of a decades-old deal with the federal government that critics say is overly favourable to the plant’s owners.

Public ire has been boiling over. In April 2017, masked protesters marched on the factory holding crosses that read “Coca-Cola kills us” and demanding that the government shut the plant down.

“When you see that institutions aren’t providing something as basic as water and sanitation, but you have this company with secure access to one of the best water sources, of course it gives you a shock,” says Fermin Reygadas, director of Cántaro Azul, an organisation that provides clean water to rural communities.

Coca-Cola executives and some outside experts say the company has been unfairly maligned for the water shortages. They blame rapid urbanisation, poor planning and a lack of government investment that has allowed the city’s infrastructure to crumble.

Climate change, scientists say, has also played a role in the failure of artesian wells that sustained San Cristóbal for generations.

“It doesn’t rain like it used to,” says Jesús Carmona, a biochemist at the local Ecosur scientific research centre, which is affiliated with the Mexican government. “Almost every day, day and night, it used to rain.”

But at a time of growing strife between Mexico and the United States, fed by President Donald Trump’s vow to build a border wall and his threats to scrap the North American Free Trade Agreement (Nafta), the increasing antipathy towards Coca-Cola has come to symbolise the frustration many Mexicans feel about their northern neighbour.

The plant is run by FEMSA, a food and beverage behemoth that owns the rights to bottle and sell Coca-Cola throughout Mexico and much of the rest of Latin America. FEMSA is one of Mexico’s most powerful companies; Vicente Fox, a former chief executive of Coca-Cola in Mexico, was the country’s president from 2000 to 2006.

Nafta has been beneficial for FEMSA, which has received hundreds of millions of dollars in foreign investment.

But in San Cristóbal, Nafta is widely viewed as an unwelcome interloper. On New Year’s Day in 1994, the day the trade pact went into effect, rebels from the Zapatista Army of National Liberation swept into San Cristóbal, declared war against the Mexican state and burned government buildings.

Although the two sides eventually signed a peace agreement, anti-globalisation sentiment still simmers across the region, one of the poorest in Mexico.

“Coca-Cola is abusive, manipulative,” says Martin López López, a local activist who has helped organise boycotts and protests against the soft drinks company. “They take our pure water, they dye it and they trick you on TV saying that it’s the spark of life. Then they take the money and go.”

FEMSA executives say the plant has little impact on the city’s water supply, noting that its wells are far deeper than the surface springs that supply local residents.

“When we hear, and when we read in the news, that we’re finishing up the water, the truth is it really shocks us,” says José Ramón Martínez, corporate affairs director for Coca-Cola FEMSA.

The company is also an important economic force in San Cristóbal, employing about 400 people and contributing around $200m to the state economy, Martínez says.

Critics, however, say the sweetheart deal between FEMSA and the federal government doesn’t serve the city well.

Laura Mebert, a social scientist at Kettering University in Michigan who has studied the conflict, says Coca-Cola pays a disproportionately small amount for its water privileges – about 10 cents per 260 gallons.

“Coca-Cola pays this money to the federal government, not the local government,” Mebert says, “while the infrastructure that serves the residents of San Cristóbal is literally crumbling.”

Among the issues facing the city is a lack of wastewater treatment, meaning that raw sewage flows directly into local waterways. Carmona, the biochemist, says San Cristóbal’s rivers were rife with E coli and other pathogens.

Last year, in an apparent effort to appease the community, FEMSA began talks with local residents to build a water treatment plant that would provide clean drinking water to 500 families in the area.

But rather than easing tensions, the plan led to more protests by locals and forced the company to halt construction of the facility.

“We’re not against the treatment plant,” says León Ávila, a professor at the Intercultural University of Chiapas, who led the protests. “We just want the government to fulfil its obligation to provide potable water for its citizens. How are we supposed to allow Coke to wash its sins after years of taking the water from San Cristóbal?”

Since bottles of Coca-Cola arrived here half a century ago, the beverage has been deeply intertwined with the local culture.

In San Juan Chamula, bottled soda anchors religious ceremonies cherished by the city’s indigenous Tzotzil population.

Inside the town’s whitewashed church, tourists step gingerly across carpets of fresh pine needles as copal incense and smoke from hundreds of candles fill the air.

But the main draw here for tourists is to watch the faithful, who pray over bottles of Coke or Pepsi, and also over live chickens, some sacrificed on the spot.

Many Tzotzil believe carbonated soda has the power to heal the sick. Mikaela Ruiz, 41, a local resident, recalls how soda helped cure her infant daughter, who was weak from vomiting and diarrhoea. The ceremony was performed by her diabetic mother, a traditional healer who has performed the soda ceremonies for more than 40 years.

But for many in San Cristóbal, the ubiquity of cheap Coca-Cola – and the diabetes that stalks nearly every household – simply compounds their anger towards the soft drink company.

Local health advocates say aggressive marketing campaigns by Coke and Pepsi, which started in the 1960s, helped embed sugary soft drinks into local religious practices, which blend Catholicism with Maya rituals. For decades, the companies produced billboards in local languages, often using models in traditional Tzotzil garb.

Although Coke has since discontinued the ad campaigns, Martínez, the FEMSA spokesman, describes them as “a gesture of respect towards indigenous communities”.

He also rejects criticism that the company’s beverages have had a negative impact on public health. Mexicans, he says, may have a genetic proclivity towards diabetes.

While scientific research does suggest that Mexicans of indigenous ancestry have higher rates of diabetes, local advocates say this puts even greater responsibility on multinational companies that sell products high in sugar.

“Indigenous people ate very simple food,” says López, the activist, who spent years living with rural communities as a missionary. “And when Coke arrived, their bodies weren’t ready for it.”

Abadía, the security guard, blames herself for drinking so much soft drinks. Still, with her mother’s health deteriorating, and having watched her father die from complications from diabetes, she can’t help but fear for her own wellbeing.

“I’m worried I’ll end up blind or without a foot or a hand,” she says. “I’m very scared.”