It’s midmorning and hundreds of people are squeezed under a banyan tree’s shady canopy to have blood drawn by just three nurses, working assembly-line fast. Others wait outside this dusty rural health center to get their vitals taken and give urine samples.

Most of the 1,000 villagers have come here on foot and have stood for hours under the hot sun — not because they feel sick, but out of fear. They want to know if they will be the next victims of a mysterious kidney disease that has killed thousands of farmers in Sri Lanka’s rice basket.

Many have watched neighbors and loved ones — some only in their 30s — quickly succumb to deaths after their kidneys gave out. In the worst-hit villages, it kills as many as 10 people every month. No one knows why.

“In some cases, you only know if a certain person died of kidney disease after the autopsy,” says Kalyani Samarasinghe, 47, standing outside the health center with a handful of medical papers and a cotton ball taped to her arm. “If you get a pain in the stomach or something, then you think: Is it the kidney?”

The disease has killed up to 20,000 people over the past 20 years and sickened another 70,000 to 400,000. It has expanded from two districts to seven in the North Central province’s dry zone, where farming was transformed after the introduction of modern techniques in the 1960s and ’70s. No cases have been detected elsewhere in the country, and research has failed to determine the cause.

Some blame the water, heavy use of pesticides and fertilizer. Others wonder if it’s something in the food, or whether heavy metals or toxic algae could be the source. Villagers have been told to give up lake fish, aluminum cooking utensils and illicit home-brewed alcohol, but no one’s sure if any of it helps.

Dr. Rajeeva Dassanayake, a kidney specialist at the area’s largest hospital in Anuradhapura, has come to the screening to try to calm the crowd.

“You need not fear and flee from this place,” he says. “There are a lot of things being said by many people. Until they have finished fighting each other and come up with an answer, we can say nothing.”

Similar diseases are wiping out thousands of farmers in parts of Central America, India and Egypt. In hard-hit Nicaragua and El Salvador, some believe agrochemicals are the problem, while others think years of prolonged dehydration in the baking heat is shutting workers’ organs down.

In Sri Lanka, a report published by the World Health Organization two years ago found kidney disease in 15 percent of adults across three affected districts. More women were affected overall, but men over 39 years old were more severely sickened. Elevated cadmium and pesticide residues were detected in urine, leading the authors to surmise they may be damaging patients’ kidneys over time, in combination with other factors such as arsenic.

Water — the source widely suspected as the culprit — came up clean. But the WHO study’s Geneva-based author, Shanthi Mendis, says it could still be playing a key role when combined with other elements. She adds that the government’s top two priorities should be supplying safe drinking water to residents and regulating the use of agrochemicals.

The WHO report documented raised levels of cadmium and lead in certain plants and vegetables, such as lotus root and tobacco. Some question whether these heavy metals could be leaching into the soil and groundwater from pesticides and fertilizers, which have been found to have high levels of cadmium in previous studies. The Agriculture Ministry says samples are regularly tested and come back within permitted limits, which are much stricter than those in neighboring countries.

Still, farmers douse their fields with too many chemicals, often using concoctions that include kerosene and multiple poisons. Most do not wear protective gear.

“Without it, it’s difficult to get rid of weeds,” says Ajith Welagedara, a backyard farmer who mixes a stronger-than-recommended amount of glyphosate, the country’s most popular weed killer, and sprays it while walking barefoot without gloves or a mask. “I’m concerned, but there’s no other way.”

The government placed a ban on some agrochemicals since the WHO report came out, but it has not been enforced and the pesticides remain available.

For those already sickened, care is often inadequate. The country of 20 million, which emerged from a quarter-century of civil war in 2009, has just 183 dialysis machines, forcing most villagers to receive less than the three recommended weekly treatments. Since no national cadaver transplant program exists, many patients post desperate newspaper ads with their photos and blood types pleading for kidney donors.

The Health Ministry is trying to screen the area to determine how many people are sick with the so-called chronic kidney disease of unknown etiology, which often strikes several members of one family.

Fear is breeding stigma, but it is also driving many villagers to be screened. They hope early detection can help slow the disease, which typically does not produce symptoms until late. But their biggest worries extend much further.

“The major fear is about the children,” says Samarasinghe, the villager at the community screening. “We have lived our lives to a certain extent, but it’s the children we’re worried about.”