I'm new at this, so I only have a couple:
1. Do NOT mention people you know who have died from the disease. I mean, Jesus, do I really even need to say this?! WTF?! I don't care how well-meaning you are. I know that you are trying to make a connection and to tell me that this issue has deeply touched your life. But that is not a connection - that is about *your* needs, not about the other person's needs. Along these same lines, do not hint about people who have not done so well, do not make jokes about the other person's potential life span, just do not allow the specter of death to enter the conversation unless the person with the illness wants to bring it up.
2. Please don't look to the person with the illness to reassure *you*. That means not asking hopeful/panicked questions about the stage of cancer, about how early the disease has been caught, etc. If you want to ask about treatment, only do so if you can take it in stride. That means *not* looking or sounding freaked out. A good rule of thumb: you probably don't know much about this person's specific situation, so don't make assumptions, and please don't look worried when someone is telling you that they're feeling very positive - that is extremely off-putting and, for anyone with anxieties, anxiety-provoking.
I'm sure that there will be more to follow.