MARECHAL RONDON BASE, Brazil — Maj. José Maria Ferreira smiled as he listed the threats to human survival in the canopied jungle enveloping this remote military outpost in the Brazilian Amazon.

He started with the piranhas, which lurk in rivers, and the pit vipers like the feared bushmaster, the Western Hemisphere’s longest venomous snake. Then he moved on to the silent creatures, including the formiga-cabo-verde, called the bullet ant in English and found in colonies at the base of trees. Its sting, according to victims, hurts about as much as being shot and lasts for a good 24 hours.

Widening his grin, Major Ferreira then described leishmaniasis, the flesh-eating disease caused by sand-fly bites, the mosquito-borne fevers like malaria and dengue and, finally, rhabdomyolysis, a condition brought on by extremely strenuous exercise. It leads to kidney damage and the breakdown of skeletal muscle tissue; victims can identify its…