Northeastern Argentina, popularly known as Mesopotamia because most of it lies between the Uruguay and Paraná rivers, is an area that boasts extensive subtropical forests and one of the country's greatest tourist attractions in Iguazu falls. Along with Buenos Aires and Patagonia's Moreno Glacier, it's one of the big three for first-time visitors. Recently, though, Argentine authorities have reported the first confirmed case of mosquito-borne yellow fever in 40 years, in the Mesopotamian province of Misiones. Bordering Paraguay and Brazil have had more cases, some of them resulting in deaths, but the Argentine victim (who was apparently unvaccinated) is recovering.
Once so widespread that an outbreak killed thousands in temperate Buenos Aires in the 1870s, yellow fever survives mostly in the humid tropics. Its presence in the region is no cause for panic, but Iguazú-bound travelers should be sure their yellow fever vaccinations are up to date. The individual in question had apparently been clearing trees in a remote area where several monkeys (presumably howler monkeys, which are common in the area) had recently been found dead.
UPDATE: As of Tuesday, March 11, there has been one yellow fever death in northeastern Argentina. Many people here in Buenos Aires are being vaccinated, but it's not really essential unless you're traveling to that region.