First, we need to understand where sickness came from. I have no doubt that sickness was not part of the pristine, sinless creation that God called "good". The atonement of Christ included a provision for healing, so it would seem that when things are put right with God, sickness is taken out of the picture. The Apocalypse clears up any doubt on the subject by revealing that in the eternal age to come, trees will provide leaves which keep the body well.
So, sickness must have come into being as a result of the negative impacts that came upon mankind due to not being right with God. That would leave, it seems to me, two possible instigating agents for sickness: God or the Devil. There are scripture passages which attribute illness to God, and passages which attribute it to the Devil, so maybe sickness is a tag team effort.
I don't believe that to be the case, but let me ask you a clarifying question that may help us discover what is the case: "Did bacteria, amoebas, and worms exist before the Fall?" My answer is yes, they most certainly did. God alone is shown scripturally to have the ability to create life, so the Devil cannot be given the credit for their existence. Yet, most of what we experience as sickness is the direct result of creatures such as these infecting our bodies.
I'm led to wonder if our body's God-given equipment (namely our immune system) worked as well as it must have by design specs called "good" by God, would humans or any life form ever get sick? If the virulence of tooth and claw existence that resulted from our separation from God, from death, and from antagonism resulting from a justified curse upon sin did not exist, would there be anything to get sick from?
Even though I could see viruses, prions, cancer and genetic defects as being the result of some devilish ingenuity manipulating what God had created, would these types of things have any ill effect (let alone exist) if we were still in the pristine physical condition we had before the Fall? No, it seems clear to me that sickness exists because of the justified curse of death upon Adam and his race as a consequence of sin. In fact, it seems quite clear to me that disease actually results from death, rather than death resulting from disease.
Our bodies were stricken with a curse which has them decaying toward death, really, from the moment they come into being. Everything in creation was stricken by God so that it opposes us and our physical existence--the ground, plants, animals, and even our bodies themselves. Our bodies do not work up to original design specs; they are infirm; they don't recover like they could; they age, and then they die. Bacteria, viruses, parasites, and genetic anomalies are a problem now, because of enmity imposed by the curse, and by our immune systems and the replication processes in our cells not working perfectly as they did in Eden.
Now that is not the Devil's doing, at least not directly by authority, it was God's doing, and God's alone. He had the authority: he spoke the curse. Does the Devil play a role in illness? Peter seemed to think he could. The Evangelists often referred to exorcism in terms of healing. It certainly appears that illness and physical affliction are tools in the Devil's arsenal against the human race (remember Job) in his antagonism toward them.
That is not, however, the same as saying disease is caused by the Devil and his demons. Disease, by general principle, is the result of the Fall, and therefore, in a very real respect, is the God-given, natural lot of life on this fallen planet. And all life is subject to it. That the devil uses what is available to him to exacerbate our condition and to increase misery should not be surprising to anyone. But to see the Devil behind every bush, or every sniffle, is a mistake, and frankly, gives him far too much credit.
Why do people get sick? Simply, because they live in decaying (dead) bodies. The Devil and his roam about seeking to add misery and to steal days from us while we're in this condition, but they are merely the exploiters, not the authors of it. If the Devil and his minions were deep-sixed today, sickness would still be a possibility tomorrow. Ultimately, to solve the problem and possibility of illness, the curse of death has to be removed from our physical being.
So what's a body to do in the meantime? That, and a few other issues in the posts to follow, starting with this one.