Over a candlelit dinner of grilled pork, coconut rice, and gingered carrots, I meet the two other guests: Keith and Helen Chapman, a couple from England who are wilderness-trekking fanatics. At seventy, Keith recently had both hips replaced. He and Helen have done the two-week Everest Base Camp Trail and marched halfway up the Andes. He had his operation so they can keep on trekking, and the idea of penetrating remote African wilderness does not deter them.

Our Tanzanian guide, Kabeth H. Kabeth, gives us the skinny on chimpanzee observation. Of the seven hundred or so chimps living within the 623-square-mile park, the fifty-nine-strong M-group—named after Mimikire, the original alpha male—has been observed for more than forty years by scientists from the University of Kyoto and are habituated to dilettantes like us. However, continual contact with humans has carried a price. The M-group's population is down from more than one hundred in the mid-nineties; in 2006 alone, twelve chimps died of a flulike ailment almost certainly contracted from humans, who share more than ninety-eight percent of their DNA.

As a result, park officials now limit chimp-watching groups to six and a guide, all of whom must stay at least ten meters from the animals. Trackers go out before we wake in the morning; look for nests, droppings, and fruit scraps; and radio back as soon as they locate chimps. Depending on where they have been feeding, reaching the chimps may involve a five-minute boat ride down the beach or a four-hour hike toward a mountain summit. Once we reach the ten-meter limit, we must don surgical masks and content ourselves with one hour of observation per day—regulations intended to reduce the risk of contagion. "Keep track of your belongings, because some of the chimps are very curious and have not heard about the ten-meter rule," Kabeth quips.

In the morning, Kabeth brings news that the trackers have located Pimu, M-group's current alpha male, a thirty-minute walk behind the camp. "Be ready to go in ten minutes," he says. "If Pimu decides to move, the others will follow. Chimps travel fast, so they might be hours away if we delay."

Carrying cameras, rain ponchos, water bottles, and our surgical masks, we enter the forest along paths covered in dead leaves, clambering over fallen branches and under huge creepers, using walking sticks to ease stiff hips and knees. Where I see a wall of green, Kabeth sees a salad bar and a pharmacy. There are primitive three-leafed cabbages, fragrant ginger, wild licorice; plants that can ease difficult births or induce abortions; and those that can relieve diarrhea, malaria, and tuberculosis. He shows us sticky-furred aspilia leaves that chimps roll and swallow whole to rid their systems of parasites, and tiny, blue, cup-shaped flowers that the Batongwe people, who once shared this forest, squeezed for eye drops. Of the more than 500 plant and tree species, Mahale chimps eat 198 and use 123 as medicines.