I ushered at a local theatre today and was at the front door taking tickets. A random woman came up, offered me her ticket to scan, then looked at me (wearing my chic little black velvet cap) and said, "God bless you." I hadn't sneezed.
One thing cancer patients experience over and over are people's inappropriate remarks. Sometimes from strangers, sometimes from family and friends, when people don't know what to say, they often say the first thing that pops into mind, instead of saying nothing at all.
I recommend reading "The Etiquette of Illness" by Susan P Halpern. Although not new, many of Halpern's suggestions from ten years ago remain pertinent. She offers many stories, comments and suggestions on how to respond, and advice from her years counseling those with serious illnesses and their families and friends.
And when you don't know what to say, and are tempted to open your mouth anyway, remember that it's okay to say nothing.