Whenever Christians talk about healing, someone inevitably asks, "Do you ever get sick?" with the implication that because a Christian believes in healing that Christians don't get sick, which obviously isn't true. I would say that on average Christians should be less sick than non-Christians for the reasons I'm about to explain, but in no way does that mean Christians don't get sick.
Where does sickness come from? In Acts 10 we read that all sickness is from the devil, and that Jesus came to heal. As we read Matthew, Mark, Luke, and John we see that Jesus healed people wherever He went, and the Bible never says that Jesus made someone sick. In fact, Jesus said that His power was opposite Satan's when He was accused of casting out devils through the prince of the devil. In other words, the pharisees accused Jesus of being in the wrong kingdom, but Jesus quickly showed them that no kingdom divided can stand, which is another proof that all sickness comes from the devil.
Now that we understand where sickness comes from, let's go back to our original question: why do Christians get sick? A natural related question would be: why does anyone get sick? The Bible has a simple answer: the wages of sin is death. (Romans 6:23)
Sin causes sickness on the earth. Now, this doesn't mean that every sick person got that way because of a sin they committed, but the general sin of the earth invites sickness because sin allows demonic powers to come, and many times they bring sickness. When the Israelites doubted God could bring them into the promise land, everyone got stuck in the wilderness for forty years, including the believers, like Joshua and Caleb. Do you remember the blind man Jesus healed in John 9? His disciples asked Him who had sinned. Jesus responded, "Neither hath this man sinned, nor his parents: but that the works of God should be made manifest in him." So, we see that all evil in the world comes from sin, either by the individual themselves or by the sins of others.
Here's an interesting passage Job 33:14-22:
14 For God speaketh once, yea twice, yet man perceiveth it not. 15 In a dream, in a vision of the night, when deep sleep falleth upon men, in slumberings upon the bed; 16 Then he openeth the ears of men, and sealeth their instruction, 17 That he may withdraw man from his purpose, and hide pride from man. 18 He keepeth back his soul from the pit, and his life from perishing by the sword. 19 He is chastened also with pain upon his bed, and the multitude of his bones with strong pain: 20 So that his life abhorreth bread, and his soul dainty meat. 21 His flesh is consumed away, that it cannot be seen; and his bones that were not seen stick out. 22 Yea, his soul draweth near unto the grave, and his life to the destroyers.
This scripture is saying that sickness or accidents can be the result of disobedience on the part of the sick or injured person. In the first few verses God is trying to speak to the person, but they choose rebellion instead. Our Heavenly Father, the great judge, then allows the devil limited access into the person's life because they invited him. The enemy is called the "god of this world" because he has some authority on the earth for a period of time. Notice I used the word "disobedience" because many times it's not blatant sin against the Ten Commandments that get us in trouble, but disobedience against what God has told us directly through dreams or other ways. Also, God always gives us time to repent.
Do you remember Jonah? Why did he end up in the belly of the great fish? Because he was running away from God. It was not a sin against the Ten Commandments or anything like that. He was simply not doing what he knew in his heart that God had told him to do. Jonah knew the grace and mercy of the Lord, and that God would forgive the people of Nineveh, but Jonah wanted them to be punished for their sins. Yet, while he was in the whale, he changed his mind, and decided to obey God. This obedience brought his salvation.
Here's another scripture to consider: Ezekiel 18:27-28
27 Again, when the wicked man turneth away from his wickedness that he hath committed, and doeth that which is lawful and right, he shall save his soul alive. 28 Because he considereth, and turneth away from all his transgressions that he hath committed, he shall surely live, he shall not die.
Basically, this says if you're sick, God will heal you when you repent of your sins. In Matthew 9, Mark 2, and Luke 5 Jesus told the man sick of the palsy his sins were forgiven, then He healed him.
Finally, Jesus healed a man in John 5, and told him, "Behold, thou art made whole: sin no more, lest a worse thing come unto thee." So, now this guy needs to keep his life straight or else something worse will come upon him. Here we see Jesus warning someone they better not sin, so they won't have that problem again or a worse problem.
As we saw in these scriptures, the devil causes sickness, and he's allowed to come into our lives when we sin. So, how do we stop sinning? Jesus told the man in John 5 to stop sinning. As we get closer and closer to Jesus through our daily worship and devotions, His power comes into us to be able to resist sin.
To receive healing: My son/daughter, attend to my words; incline thine ear unto my sayings. 21 Let them not depart from thine eyes; keep them in the midst of thine heart. 22 For they are life unto those that find them, and health to all their flesh. Proverbs 4:20-22
If you read, hear, and meditate the Word of God all day, the healing power of Christ will flow into your body so you can receive your healing.
Here's a link to our Healing Scriptures.