The 5 most common hospital-acquired infections cost the U.S. health care system almost $10B a year, according to a new study by Harvard researchers. The study was published online in JAMA Internal Medicine this week.  According to CDC, one out of every 20 patients admitted to a hospital will pick up an infection while there.  Central line-associated bloodstream infections averaged about $45,000 per case. Pneumonia infections that strike patients who are put on ventilators to help them breathe cost about $40,000 per case. The most common infections, surgical site infections, which happen in about one out of every 50 operations, cost around $21,000 each to treat.