A reader wrote to us on behalf of a friend who is believed to have a “worm in his foot.” The foot worm, to give the creature a concise and descriptive name, is only visible some of the time, and by “visible” we mean that a bulge can be seen on the reader’s friend’s foot, not that the worm itself can be seen. The victim of the foot worm went to a doctor, but the worm “disappeared” during the visit, which left the doctor unable to extract the worm from the foot of our reader’s friend. What exactly is this foot worm, and what kind of damage can it do?