American Peggy Lillis thought she had a stomach virus. But her condition was much worse than she thought.She had become infected with what researchers call a "superbug" -- clostridium difficile-related infection or "C. difficile."It is one of the fastest-growing superbugs. Ms. Lillis had been given antibiotics after receiving care for her teeth. Doctors believe the medicine killed good bacteria in her body.They believe this made it easy for her to be infected with C. difficile. She died within days of being infected.Fifteen-year-old Nile Moss of California went to a hospital for medical tests. Within a few days, he died of pneumonia.The young man had become sick with methicillin-resistant staphylococcus aureus, or MRSA.It is known as the "hospital superbug," because people who get MRSA are often infected with it when they are in a hospital.Joshua Nahum went to the hospital after he was injured while sky-diving. His health started to improve during his hospitalization.