Paraguay has eliminated malaria, the first country in the Americas to do so in almost 50 years, according to the World Health Organization.

But worldwide, momentum against the disease has stalled. Malaria cases increased by 5 million between 2015 and 2016, climbing to 216 million from 211 million.

Nine countries in the Americas reported at least a 20 percent increase in malaria cases during that period — greater than in any other region.

“This is one of the diseases that hangs on tight,” said Luis Alberto Moreno, the president of the Inter-American Development Bank, which finances anti-malaria efforts in the Americas. “If you don’t keep the pedal to the metal — stay intensely focused on the issue — malaria is going to make its return.”

Malaria, a blood disease contracted through the bite of an infected mosquito, kills about 445,000 people each year, mostly children, the WHO said. Yet cost-effective prevention tools and treatments are well known.

Public health officials at the first Malaria World Congress attributed Paraguay’s success to the national health system’s ability to quickly detect cases and investigate whether they had been transmitted locally or imported. WHO officials also expect to certify Argentina as malaria-free later this year, said Dr. Marcos A. Espinal, director of the communicable diseases department at the Pan American Health Organization.

But throughout the region, other countries are backsliding. Panama, Nicaragua, Peru and Venezuela each reported more malaria infections in 2016 than in 2010. Cases in Colombia doubled from 2015 to 2016. Officials say the chief hurdle is complacency: In many countries, domestic resources have shifted from anti-malaria efforts to other priorities as case numbers have dwindled.

“Political will is the single most important aspect for eliminating malaria,” Espinal said. “We have effective tools: bed nets, vector control methods, treatments. We get to a certain point — we see the end of the tunnel — and then we risk losing the commitment.”