Scientific experimentation using animals—including mice, rats, rabbits, guinea pigs, monkeys, and dogs—dates back to ancient times. But the practice did not become widespread until the late nineteenth century. Clinical experimentation, which includes vivisection (surgery on live animals), has yielded benefits to human health, but because it often results in the suffering and death of the animals, many people are against the practice. Tens of millions of animals are used for experimentation in the United States today; official estimates cite that mice and rats account for some 90 percent of this number. The practice remains controversial as people grapple with the issues surrounding animal rights and weigh these considerations against improved scientific understanding of illnesses.