I haven't read the this book. The only book any Christian should follow the teachings of is the bible.
Here is what I think about sickness and where it comes from:
1 Corinthians 11:30-32
That is why many among you are weak and sick, and a number of you have fallen asleep. But if we judged ourselves, we would not come under judgment. When we are judged by the Lord, we are being disciplined so that we will not be condemned with the world.
John 5:14
Later Jesus found him at the temple and said to him, "See, you are well again. Stop sinning or something worse may happen to you."
As you can see from the above, God can punish those who continue to sin by making them sick or even causing them to die if they further continue to sin still.
We must keep these scriptures in mind in relation to this however:
Hebrews 12:4-13
4In your struggle against sin, you have not yet resisted to the point of shedding your blood. 5And you have forgotten that word of encouragement that addresses you as sons:
My son, do not make light of the Lords discipline,
and do not lose heart when he rebukes you,
6because the Lord disciplines those he loves,
and he punishes everyone he accepts as a son.
7Endure hardship as discipline; God is treating you as sons. For what son is not disciplined by his father? 8If you are not disciplined (and everyone undergoes discipline), then you are illegitimate children and not true sons. 9Moreover, we have all had human fathers who disciplined us and we respected them for it. How much more should we submit to the Father of our spirits and live! 10Our fathers disciplined us for a little while as they thought best; but God disciplines us for our good, that we may share in his holiness. 11No discipline seems pleasant at the time, but painful. Later on, however, it produces a harvest of righteousness and peace for those who have been trained by it.
12Therefore, strengthen your feeble arms and weak knees. 13Make level paths for your feet, so that the lame may not be disabled, but rather healed.
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But this is one-sided when looking into "teachings" as you put it, you cannot just concentrate on one or two scriptures, you need to take all the related ones into account.
John 9:2
His disciples asked him, "Rabbi, who sinned, this man or his parents, that he was born blind?" "Neither this man nor his parents sinned," said Jesus, "but this happened so that the work of God might be displayed in his life.
John 11:15
So then he told them plainly, "Lazarus is dead, and for your sake I am glad I was not there, so that you may believe."
Therefore, as you can see from the above, people have in the past been sick so that God can get glory from it.
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Let's look at it from another perspective now.
Luke 13:11
And there was a woman who for eighteen years had had a sickness caused by a spirit; and she was bent double, and could not straighten up at all.
Luke 13:16
Then should not this woman, a daughter of Abraham, whom Satan has kept bound for eighteen long years, be set free on the Sabbath day from what bound her?"
This tells me that we can also get sick from spirits (but only because God has allowed it for he has power over them).
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It's important to also keep this scripture in mind as well:
Romans 8:28
And we know that in all things God works for the good of those who love him, who have been called according to his purpose.
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Therefore, my conclusion is that a person can get sick for any of the above reasons. But when you do get sick, ask yourself if you are being punished, ask yourself if you are living in sin? Think about Jonah and the whale, he had to realise what he was doing was against Gods will, he repented and was set free from the belly of the fish, then he did what God wanted him to do.
