Miracles can cure anything, at least in theory. In real life these are not as obvious or common as roleplaying games. While heroes often possess great power, and the function of a Cleric in-game terms is to reset things to enable continued game play. However that seems overpowered when you ask the question, why would there be any sickness left in the world if you could just heal it away. Here are some possible explanations.
- Heroes are fit, able bodies, and as such they are like high performing athletes that have access to good medicine and training regimens etc. So they are able to bound back faster than the standard populace when healed.
- Healing restores vitality, fixes cuts and bruises but only temporarily causes sickness to flee. So for the flu it works great, but for diabetes it only can provide temporary relief. There is not enough healing power to provide constant healing for much of the wider elderly population. Lesser and Greater restoration are usable but only in some narrow confines, which come up a lot in adventuring but not in mundane life.