Guinea worm is a debilitating parasitic disease that in the mid-1980s used to infect millions in the developing world. But thanks to the work of the Carter Center, there were only 22 cases of guinea worm in 2015, a stunning public health victory.

The Carter Center, the NGO that has led the charge to eradicate guinea worm, says that's an 83 percent decline from the 126 cases it reported in 2014.

There's no cure or vaccine for guinea worm — so the Carter Center focused on prevention

The treatment for guinea worm hasn't changed in thousands of years — and it's awful.

Once the worm is exposed, treatment consists of gradually coaxing it out of the affected person, a process that, given that worms can be as long as a meter, can take days if not weeks. It's a hugely time-consuming and painful process. Other than antibiotics to prevent infection at the site of the wound, and painkillers to ameliorate the pain of the process, there's little drugs can do to help.

So if you can't vaccinate people or offer drugs to kill the worm larvae before they grow into worms, breed, and cause intense pain and debilitation, how has the disease been cut back so dramatically?

According to Donald Hopkins, vice president for health programs at the Carter Center, a major tactic that made eradication feasible was public health education. The key problem was that people with the worm were submerging affected body parts into the same water sources that other people would drink. That would create a cycle of infection.

The cycle was broken by both taking steps to purify water sources and conducting public education campaigns about the importance of not soaking guinea worm–affected body parts in a public water source. Dr. Michele Barry explained how the last works in practice in the New England Journal of Medicine:

In Uganda, the eradication program has employed elderly men as "pond caretakers" to guard ponds against contamination by worms emerging from people. When infected people are identified at a pond, the caretakers assist them with water gathering, preventing contamination of the water, and distribute nylon filters for ongoing prevention. Cash rewards are sometimes offered to those who report cases or to infected villagers who agree to be quarantined while the worm is emerging; often such persons receive free care and food during that period.

It was remarkably cheap

Hopkins estimates the total cost of the three-decade eradication campaign at about $350 million. Any way you slice it, that's a bargain given the amount of human suffering averted. A 2011 Center for Global Development case study pegged the cost at $5 to $8 per person treated; a 1997 analysis by the World Bank estimated that if one only considers the increase in agricultural productivity that would result from eradicating the worm (achieved by eliminating worm-related work absences), and no other potential benefits, the rate of return is 29 percent.

In other words, for every $1 invested in the campaign, $1.29 came back in the form of greater agricultural output and earnings.

Even better, the campaign comes with a number of positive externalities that help accomplish other global development tasks. There's the increase in agricultural productivity, for one thing, but additionally, researchers at the Carter Center and Emory point out that school attendance can tick up when an area is rid of guinea worm.

They cite a study in Nigeria that found that villages given wells (providing a source of clean, worm-free drinking water) saw the incidence of the disease fall 62.5 percent as attendance at school rose 50 percent and enrollment went up by 12 percent.

-Vox