The Aria Hotel and Casino recently sent letters to its guests, telling them they may have been exposed to the bacteria causing Legionnaires disease between June 21-July 4. Six former Aria guests reported being diagnosed with Legionnaires disease. So what is Legionnaires? How did it get into Aria's water system? And why is the hotel sending out warning letters now, when the CDC notified Southern Nevada health officials of the potential problem back in 2010? A Southern Nevada Health District expert joins us with some answers.