I find myself agreeing with Holly and then I read posts like Tourist's and I'm like, "Wow, I wish I could be that kind of person, too!"
I dated a single dad with severe alcoholism for 3 years. I know this isn't necessarily a terminal disease or mental illness, but I can tell you from my experience that I wound up using all my resources to pay for him and his children while he sat around in a drunken stupor. OF COURSE not every situation is like this but I was very naive going into the situation. I even thought it might be God's calling for me to care for his kids.
Allow me to be blunt. The entire situation was a living hell. I'm not saying this to discourage anyone, but to be honest. If you are faced with the decision of whether or not to enter into a relationship that includes these issues, find out as much as you can about the issue, talk to people who have dealt with it, and make sure you are able to take care of yourself as well and have a good support network.
After this experience, I think I could be friends with someone who has these issues, limiting my support to friendship only, but would most likely not choose to date them.
I also had a friend several years ago who, in her 20's, was engaged to a young man who had leukemia. As heartbreaking as it was, his treatment was not successful and she was there with him, holding his hand when he took his last breath in the hospital.
But, as others have pointed out, we will all die someday and we never know when tragedy may strike.
So... I can't say "never" with absolute certainty. God has this way of changing my mind about things...