A good starting point on this issue is a little kuntres of the Chazon Ish printed in the back of the Taharos section and reprinted as a separate sefer entitled Emunah u'Bitachon. The C.I. contrasts emunah, which means theoretical belief in G-d, with bitachon, which is the day to day implementation of emunah. Emunah tells us that G-d is in charge of the world; bitachon tells us how to live with that knowledge. Bitachon does not mean that G-d will not make a person suffer illness, unemployment, or other tragedy; it simply means that a person who is suffering is allowed to make whatever the normal efforts to relieve that situation are and then trust that whatever ultimately happens is due to G-d's plan. I think this summary by R' Hershel Shachter shlit"a sums it up well:
The Chazon Ish explains in his essay on Emunah and Bitachon, that when we ask a sick person to have bitachon, it does not mean that he should be convinced that he will recover. That would be ridiculous – one can not be sure that he will not die. Bitachon simply means to live by emunah, and emunah means believing that G-d has complete control over everything in the world. If G-d wants me to live and be healthy and happy, then there is nothing anyone can do to negate that. If for some reason, G-d wants me to suffer, then as that is His will, we should accept it with joy, with the knowledge that anything G-d does is for the good.So, to respond to a comment yesterday which raised the question, "How should I have trust in Hashem that all will be well?", the answer is, at least according to the Chazon Ish, that you should not! There is no such guarantee that all will be well. All we can say is that whatever happens is directed by G-d, but we cannot say that things will be pleasant for us along the way or that we will get the hoped for outcome just because we want it to happen and trust Hashem to make it so.
(As an aside, compare the Chazon Ish's position with Kierkegaard's knight of infinite resignation in his Fear and Trembling.)
The story does not end there... more to come bl"n.