I've had success with trying to steer them into the thinking that they'll probably be just fine and if not, it really isn't as huge of a deal since we're all going to die. On top of that getting sick has a whole lot to do with luck and genetics so they should try to accept that no matter what they do it isn't going to be up to them in most cases anyway. Lastly, I help them focus on the great people around them instead of continuing to dwell on whether or not they have optimum health.
Of course, you can't just call them a selfish jerk, but instead try to guide them out of that loop of self obsession.
Hope you can work it out or work around it, stranger things have happened.
Just nod and learn the phrase "yes dear"
(Though it's very difficult to make people actually use this in decision-making.)