James MacDonald, JSTOR Daily:

Starting in the 1980s, researchers noticed that ill chimpanzees consumed either whole, bristly leaves or the pith, or parenchyma, of a bitter African plant. Neither part of the plant offered anything valuable by way of nutrition, but affected individuals often recovered from their illness. Closer examination of feces showed that following digestion, large quantities of worms and parasite eggs seemed to have been removed. The medicinal plants were purging parasites from the digestive tract, resulting in rapid health improvement.