Anderson Didier, 5, hugs his balloon while receiving treatment for cholera symptoms at the Samaritan's Purse cholera treatment facility in Cabaret. (Nikki Kahn/The Washington Post)

THE FIGHT against the cholera epidemic in Haiti, by far the world’s worst in recent years, has been a hard slog. Still, the number of new cases had fallen precipitously, to just 1,000 per month for much of 2014 from an average in 2011 of nearly 30,000 per month.

But a recent spike — to about 1,000 new cases per week — is a grim reminder of how much is left to do to eradicate an illness that was virtually unknown in Haiti until U.N. peacekeepers from Nepal introduced it in 2010.

The surge in new cases also casts an unflattering spotlight on international donors, whose focus has gradually shifted elsewhere since the deadly 2010 earthquake killed at least 160,000 people .

It’s impossible to know whether flagging contributions reflect donor fatigue or the fact that relatively few cholera victims end up dying (less than 1 percent), thanks to quicker recognition and treatment in many parts of the country. Still, tighter money means longer odds for tackling the disease over the long term.

A plan to eliminate cholera in Haiti by 2022, devised in coordination with the Port-au-Prince government, was pegged to cost $2.2 billion. But of the $1.7 billion sought to execute the first five years of the plan, from 2013 to 2018, only 17 percent — about $286 million — has been raised and spent so far.

That means that blueprints to improve and replace portions of Haiti’s glaringly inadequate water and sanitation infrastructure are not being implemented. In the absence of those upgrades, more Haitians will continue to succumb to cholera, a diarrheal illness caused by consuming contaminated food and water.

Vaccinations have been a major focus of international health organizations combating cholera in Haiti. Yet in a country of more than 10 million people, fewer than 400,000 Haitians have received the cholera vaccine despite the efforts of organizations such as Partners in Health, which has vaccinated thousands of people in rural areas, and a Haitian group called GHESKIO, which has done similar work in the slums of Port-au-Prince. Supplies of the vaccine, which was not in wide demand before the Haitian outbreak, remain limited.

The United Nations has done extensive and admirable work in Haiti, including on public health, but it maintains it is immune from legal liability for the cholera epidemic. This is despite the consensus of health experts that U.N. peacekeepers introduced the disease into the country. In January, a federal judge in New York sided with the United Nations .

Nonetheless, it has a moral obligation to do more, including pressing donors to fund the plan to eradicate the disease. There is no mystery about how cholera is transmitted or about the means to eradicate it. Only money is lacking.