According to Watchman Nee, the late Chinese church leader and Christian leader, believers can become sick because of a variety of reasons.
Writing in his book “Messages for Building Up New Believers, Volume 2”, Nee noted that individualism, disobedience and physical defilement in believers can lead to sickness:
Sometimes there is another reason for illness—individualism. Please remember that individualism is the biggest source of illness. Some people are individualistic; they do things according to their own will. They act by themselves and are always on their own. When God disciplines them, they become ill, and the supply of the Body no longer reaches them. If this is the reason for a person’s illness, he needs to ask for the supply of the Body to flow through him once again.
I dare not say that I have listed all of the reasons for our sicknesses. There are many causes for sickness. Some become sick because they have disobeyed a command of the Lord. Others become sick because they have not fully carried out the Lord’s command. Still others become sick because of a certain sin or because of their individualism. Some are individualistic, yet God does not discipline them. However, there are many people, especially those who know the church, who become ill as soon as they become individualistic. Those who do not know the church have less illness. The more we know about the church and the more we are related to the church, the more the Lord will put us in a situation where His hand will be heavy upon us as soon as we become individualistic.
Sometimes a person becomes sick because he has defiled his body. Whoever defiles his body will find God destroying His temple in that person.
There are many causes for illness, and I cannot give a list of all of them. I can say only that there is always a reason behind any illness. We must always find the cause of our illnesses. There may be one or more than one reason. Once you discover these reasons, you must confess them one by one to God. Then call for the responsible ones of the church, confess to them, and pray together with them. Ask them to anoint you and to recover you to the life of the Body. You will find your illness immediately gone when life begins to flow through you once again. Although there are natural causes for sickness, you must know that spiritual reasons far outweigh natural reasons. Once you take care of the spiritual reasons, the illnesses will leave promptly.