On one level, of course, medical experts will sometimes be able to provide ‘reasons’ why an illness occurs: genetic or environmental factors may be involved; there may be a variety of physical causes at work in and upon a person. But we often ask this on a ‘spiritual’ level. We wonder whether, in some mysterious sense, there is a supernatural reason for our current difficulties.
This question sometimes arises because of a widespread misunderstanding of the Bible Book of Job. Some suggest that God made Job ill in order to bring him to his knees —in order that a fresh relationship between them could begin. On that view, God had a purpose in seeing Job become sick, and it would then follow that we, too, can believe that we are experiencing sickness sent by God for our own ultimate good. However this is a serious misapplication of the Job account. God has shown us his nature throughout both the Old and New Testaments as the One who heals; his will is to heal and not to hurt.
Importantly, God did in fact heal Job —a point which seems to escape most people who know of his story. In the end, Job lived to be a hundred and forty years old and was greatly blessed by God after his time of troubles.
There are always reasons of various kinds for our getting into difficulties but the cause of our troubles is never God.