If you are dealing with a respiratory infection and you have them in a rack and are using a watering system this is not good news.

You can always try to separate the sick ones placing them as far as possible and do a complete cleaning, this includes the rack, tubs and watering system (must be flushed disinfected etc)

Now keep in mind that rats do sneeze to but there is a line between normal sneeze and one due to respiratory issue.