Two patients are dead at UCLA's Ronald Reagan Medical Center. The cause: a deadly infection from a bacteria known as CRE. Almost 180 more people may have been exposed since October, too.

It was spread by two endoscopes -- devices inserted down the throat to treat various conditions. It's believed the bacteria survived the disinfection process before they were used on other patients.

Dr. Louise-Marie Dembry, hospital epidemiologist at Yale-New Haven Hospital and president-elect of the Society for Healthcare Epidemiology of America, explains why the disinfecting science and procedures in hospitals won't ever be 100 percent effective.