We are alive as long as we are aware of our physical bodies. At death, the awareness of the physical body is completely taken away.
Our sickness is an attempt of nature to bring our awareness back to the body. Any pain or uneasiness in the body make us aware of the body, thus keeps you alive.
If you want to stay healthy and away from sickness, meditate on the physical body, try be aware of each and every molecule of the body as much as possible. Thus nature will not need to make you aware of the body as you already are.
If you are sick you will recover, no matter how grave the illness.