Can knowing about illness affect mental health?

The crucial issue is whether discovering a disease will lead to treatment that can lower the death rate. If there is no benefit to improved quality of life or longevity, then such knowledge may possibly do more harm than good. The personality type of the particular person is important as some people are natural 'worriers' or hypochondriacs, while some are better able to tolerate bad news.

Early detection is extremely important and the values can not be overstated.

Yes, it can affect someone's mental health. It really comes down to the foundation the person grew up with and how it interconnects with their spirituality, if they believe in it. I've lost a good friend last year to cancer. She had such a strong connection to her religion and belief in God. When she was diagnose with a terminal form of cancer, she wasn't bitter. She felt it was her time and she was ready to move on to be with God. She always had this positive outlook on life, even when the hardest moments nearly broke her. Such an amazing woman.