Are incurable diseases curses from the almighty Allah?

Today I visited a relative's house. She is suffering from cancer. I have seen him last year during the Eid-ul-fitr. She is almost 70 now. She is so ill that she is looking like a living dead. After seeing her, I became very much unstable mentally. I am in so much fear that, the question came in my mind. Are incurable diseases curses from the almighty Allah? Why people have incurable diseases? Please explain according to Koran and Hadiths.