Cystic Fibrosis - contagious to each other.

Two cystic fibrosis patients should not come within three feet of each other if they can help it. Apparently, cystic fibrosis is not contagious, but two people with cystic fibrosis can pass on fatal infections to each other at the drop of a hat where a healthy person would not cause harm to a cystic fibrosis patient.

source - "Grey's Anatomy" of course.