For the sand flies, the decisive factor was microclimate — temperature, especially nighttime temperature, and humidity in the cracks in adobe walls where the insects live. Higher temperatures speed up incubation and hatching and increase the longevity of the adult flies. But that does not necessarily mean that a warmer planet will see an uncontrolled explosion of infectious disease, Laughlin said.
As the insect population grows, so will the number of predators, he said. And temperatures may eventually rise to a point at which the insect cannot live, changing the geographic patterns of the disease.
People change those patterns, too. From beneath the palm trees in Caraz's main plaza there is a postcard view of Mount Huandoy, one of the high points of the Cordillera Blanca. In the Andes, snow-capped mountains are both protective spirits and vital sources of fresh water for crops and villages.
Peru has the largest expanse of tropical glaciers in the world, but between 1987 and 1997 they shrank by about 15 percent, a trend that is picking up speed throughout the Andes. In Ecuador's capital, Quito, and the cities of La Paz and El Alto, on the Bolivian Altiplano, millions of people depend on water supplies fed by retreating glaciers.
Experts disagree on how soon the water crunch will turn into a water crisis. Nevertheless, Corvalán, the doctor in Brazil, expects the effects to ripple through the health system.
"If we're not feeling it now, I'm sure we're going to be feeling it very soon," he said. "This is a problem, because the health issues become very complex."
If clean water sources dry up, people re-use water or turn to unsafe sources. They also store water in containers, creating breeding grounds for disease-bearing mosquitoes. And if their crops fail — because of drought or plant diseases, which also respond to a changing climate — they may be forced to move to the city.
"It's a constant trickle," Corvalán said. "Families migrate, they disappear in the system. It is very difficult to track what is happening to them."
In urban shantytowns, people often lack running water or sewer service, and the stress of poverty and unemployment can lead to alcoholism, domestic violence and mental illness, Corvalán said. Those who try their luck in the United States may unwittingly introduce tropical diseases there. In December 2006, the U.S. Food and Drug Administration approved a test to detect Chagas' disease, caused by a tropical parasite that affects the heart, in the U.S. blood supply.
Peru's largest cities are on the arid west side of the Andes, and water systems are already insufficient to meet the growing demand spurred by migration. Projects are already in the works to build channels from sources in the sparsely populated but water-rich Amazon basin, while tropical regions are seeing increased migration from the highlands. That will likely increase deforestation, further disrupting the hydrological cycle, which could exacerbate the water crisis.
The cycle won't end there.