I've now heard both that the respiratory disease isn't contagious and that it is - so it would be safer to separate the sick ducks, if it were a respiratory infection. Sorry for the confusion. Sneezing generally seems to come first.
It might be a respiratory problem, or it could be caused by over breeding or debris being in the eye. It's not contagious, and putting Apple cider vinegar and spraying with Vetricyn should help a lot, and hopefully clear it up. Here's a couple links to more info...