When we fall ill we visit a clinic or a pharmacy. Our ancestors, however, didn't have that luxury. Instead, early humans likely observed and learned from sick animals that healed themselves by eating certain plants. Yet, only in the past two decades have biologists and chemists begun to recognize that animals do self-medicate – select and use substances specifically to cure themselves of parasites and ailments. Early accounts of animal self-medication came in the late 1980s from Michael Huffman, a primatologist at Kyoto University. His decades-long research on chimpanzees, which revealed that they use plant compounds to rid themselves of parasites, helped established self-medication as a fundamental animal behavior. “Any animal species alive today is alive in part because of its ability to adapt and to fight off diseases,” Huffman says. Self-medication does not require high intelligence, but was simply the reaction of animals to remove an ailing symptom that evolved into strategies to expel parasites. “Self-medication is a very basic behavior that’s important to the survival of so many species,” he says. And animal self-medication points to a treasure larger than mere fascination. By following the animals’ lead, we tap into a medicine vault furnished by millions of years of natural selection. The world's best bio-prospectors – the animals themselves – may very well show us new pharmaceuticals to improve the health of our livestock and ourselves.