Sickness and death are inevitable. As long as there is birth, there is sickness and death. An example of sickness in the suttas is as follows:
(2) And further, monks, a monk reflects thus: "I am now free from sickness, free from disease, my digestive power functions smoothly, my constitution is not too cool and not too hot, it is balanced and fit for making effort. But a time will come when this body will be in the grip of sickness. And one who is sick cannot easily contemplate upon the Teachings of the Buddha; it is not easy for him, to live in the wilderness or a forest or jungle, or in secluded dwellings. Before this undesirable condition, so unpleasant and disagreeable, approaches me, prior to that, let me muster my energy for achieving the unachieved, for attaining the unattained, for realizing the unrealized, so that, in the possession of that state, I shall live happily even in sickness."
— AN 5:78
Sickness is recognized as an inevitability, and we are offered advice for how to conduct ourselves when we are healthy, and not merely be 'grateful' for not being sick.