Dr. Mark Plotkin is an ethnobotanist and conservationist who works in the Neotropic ecozone to identify and catalog natural cures derived from plants, and to advocate for preservation of the rainforests. He has written two books about natural cures and how he found them in indigenous tribes: Tales of a Shaman's Apprentice in 1994, and Medicine Quest in 2001.
Dr. Plotkin focuses on how people living in low-technological areas use natural medicines derived directly from plants instead of synthesized or chemically distilled. Because of the incredible diversity of plant life in the neotropic ecozone (which includes the Amazon jungle) Dr. Plotkin has found many plants with useful medicinal properties. Many synthetic chemicals used in medicine are duplicated in plant alkaloids, which stimulate the body in different ways. Quinine, the most famous malaria cure, is found in cinchona tree bark. The Madagascar Periwinkle is being investigated for chemicals that can help fight leukemia. Pau d'arco bark from the Amazon may help fight HIV and AIDS, while cat's claw vine is indicated for many diseases including Alzheimer's, depression, and cancer.
Rain-tree.com, linked below, has an enormous database of rainforest plants used for disease control, and both of Dr. Plotkin's books are available on Amazon.com (no pun intended).