A city is very much like a living organism. Cities are born, cities grow, cities age, cities become ill, cities heal, and cities regenerate. Although cities rarely die, elements of a city often die, and they are sometimes reborn. In the healthiest of cities, elements that become ill are quickly healed, and elements that decay and die are quickly replaced. In less healthy cities, buildings and properties that become vacant often remain in a decayed condition for many years,