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Physical suffering resulting from some sort of injury or disease, experienced through the central nervous system.

Pain is a complex phenomenon that scientists are still struggling to understand. Its purpose is to alert the body of damage or danger to its system, yet scientists do not fully understand the level and intensity of pain sometimes experienced by people. Long-lasting, severe pain does not serve the same purpose as acute pain, which triggers an immediate physical response. Pain that persists without diminishing over long periods of time is known as chronic pain. It is estimated that almost one-third of all Americans suffer from some form of chronic pain. Of these, 70 million have back pain, 36 million have arthritis, 20 million suffer from migraine headaches, and at least 800,000 Americans suffer severe pain associated with the growth of cancerous tumors. An additional kind of pain is...