I think sickness can come from many things: Satan, sin, unhealthy life style, stress, anxiety, and even God for His purposes.
Speaking from personal experience, when I was diagnosed with cancer I could not believe it. I had given my life into the hands of God, was teaching two bible classes a week, giving thanks daily for my good health, believed that His anointing and life within me was keeping me from many diseases. I had no stress or worries. I lived in the bible and in fellowship with Christ daily. I was not convicted of any particular sin and knew that my righteousness was only a free gift from His grace. I moved in the gift of healing and prayed much for others who were sick.
So, at first I felt betrayed in a way. What was this? But in prayer one day the Lord spoke to my spirit and asked if I would go through this with Him - that it would all lead to His glory and my spiritual growth in ways I could not see then. I knew then that He had allowed this to happen to me. And, yes, He has shown His hand in miraculous ways and caused me to be a miracle by the way in which I have handled all the complications of the removal of half of my mouth and 37 radiation treatments on the glands in my neck which were cancerous also. I have experienced so much more of the Lord's comfort and presence to me than in the past.
God does allow and even might bring about a person's illness for His own purposes. We can never know for sure the why of any disease and the cause, but it helps to talk to the Lord about it and then listen for His response to your spirit. I do believe that if we have surrendered our lives into His keeping, then we must believe that EVERYTHING that comes to us is from His hand.