The number of organisms, usually people, living within a defined space, such as a city, county, or town. In the U.S., regions with greater population densities tend to have different health care problems than lightly populated ones. Conditions such as gunshot wounds, HIV/AIDS, and tuberculosis are more common in metropolitan areas than in rural areas. Urban living confers some advantages, however, in that densely populated areas tend to have a greater health care infrastructure and more professional resources than rural areas.