Urgent care centers treat the most common injuries and illnesses—including colds, ear infections, cuts and back pain—in addition to taking X-rays and performing simple blood, urine and drug tests. And they are booming: An estimated 3 million patients visit them each week, according to the Urgent Care Association of America. Since 2008, the number of facilities has increased from 8,000 to 9,300. That total includes about 400 in the District, Maryland and Virginia—and that's not counting smaller and more limited walk-in clinics in pharmacies and big-box stores such as Target and Wal-Mart. Some physicians groups warn that the overreliance on the centers can complicate efforts to improve health through bette r coordination of care.