LAS VEGAS (CBS) — Aria Resort and Casino is warning guests who stayed at the hotel during a two-week period that they may have been exposed to high levels of the bacteria that causes Legionnaires’ disease.

Six guests who stayed at the hotel between June 21 and July 4 came down with the illness, which is a potentially fatal form of pneumonia.

They have already been treated.

Hotel officials say recent tests show that the water is safe.