A recent study highlights the cultural and economic forces spurring the over-prescription of antibiotic drugs, and the evolution of super-resistant microbes. The report notes that antibiotics cost the U.S. about $15 billion per year. Extra hospital costs associated with drug-resistant hospital-acquired bacterial infections total at least $1.3 billion annually. Of 51 million visits to physicians for "colds," upper-respiratory infections and bronchitis, 50% to 66% resulted in an antibiotic prescription, even though these conditions usually do not require antibiotics. To explain this trend, the study's authors point to direct-to-consumer advertising, consumer demand, a medical training system that puts the least experienced doctors in charge of prescribing drugs and overloaded hospitals.