Here's what I believe but cannot prove: human beings, like all animals, have evolved a range of capacities for fighting disease and recovering from injury, including a variety of 'sickness behaviors'; humans beings alone however have discovered the advantages of off-loading much of the responsibility for managing their sickness behaviors to other people; the result is that for human beings the very nature of illness has changed—human illness is now largely a social phenomenon.
This is possible because "illness" is a response. A rise in body temperature, for example, kills many bacteria and changes the membrane properties of cells so viruses cannot replicate. The pain of a broken bone or weak heart makes sure we let it heal or rest. Nature supplied our bodies in this way with a first-aid kit but unfortunately like many medicines their "treatments" are unpleasant. That unpleasantness, not the dysfunction which they seek to remedy is what we call "illness".
These remedies, however, have costs as well as benefits making it often difficult for the body to know whether to deploy them. A fever might fight an infection but if the body lacks sufficient energy stores, the fever might kill. The body therefore must make a decision whether the gain of clearing the infection merits the risk. Complicating that decision is that the body is blind, for example, to whether it faces a mild or a life-threatening virus. The body thus deploys its treatments in a precautionary manner. If only one in ten fevers actually clears an infection that would kill, it makes sense to tolerate the cost of the other nine. Most of the body's capacities for fighting disease and repairing injury are deployed in this precautionary way. We feel pain in a broken limb so we treat it over protectively—in nine occasions out of ten we could get by with less protective pain but on the tenth it stops us causing it further injury. But precautionary deployment is costly. Evolution therefore has put the evaluation of such deployment under the control of the brain in attempt to keep their use to a minimum.
But the brain on its own often lacks the experience to know our own condition. Fortunately, other people can, particularly those that have studied health and illness.
Human evolution therefore changed illness by offloading decisions about deployment whenever possible on to professionals. People that make themselves experienced in disease and injury, after all, have the background knowledge to know our bodies much better than ourselves. Healing professionals—healers, shamans, witch doctors and medics—exist in all human cultures. Of course, such professionals were seen by their patients as offering real treatments—and a few did help such as advising rest, eating well and some medicinal herbs. But most of what they did was ineffective. Doctors indeed had to wait until 1908 and Paul Ehrlich's discovery of Salvarsan for treating syphilis before they had a really effective treatment for a major disease. Nonetheless earlier doctors and healers were considered by themselves and their patients to be in the possession of very powerful cures.
Why? The answer I believe was that their ineffective rituals and potions actually worked. Evolution prepared us to offload control of our abilities to fight disease and heal injuries to those that knew more than us. The rituals and quackery of healers might have not worked but they certainly made a patient feel they were in the hands of an expert. That gave a healer great power over their patient. As noted, many of the body's own "treatments" are used on a precautionary basis so they can be stopped without harm. A healer could do this by applying an impressive "cure" that persuaded the body that its own "treatments" were no longer needed. The body would trust its healer and halt its own efforts and so the "illness". The patient as a result would feel much better, if not cured. Human evolution therefore made doctoring more than just a science and a question of prescribing the right treatment. It made it also an art by which a doctor persuades the patient's body to offload its decision making onto them.