In the Middle Ages, what was disease? Everyone agreed that health was wholeness: it was the body functioning as it ought to. I guess our contemporary writers who talk about “wellness” are taking a similar view. So loss of health meant that something had been taken away; it needed to be restored.
Loss of health had two primary, concrete causes: imbalance and some kind of attack, a direct theft of health.
Imbalance of humors was the Greco-Roman theory, imported to Northern Europe first with the Roman legions, then with the monastic tradition that was heavily based on Latin sources. Their answer to the question “why am I sick?” was that you ingested some substance that imbalanced your system, or else you have an inherent humor-balancing problem.
The substance was probably food: you ate too many cold and wet foods, which cooled the fires of the stomach, so that food was not properly digested. The body tried to remedy the situation by heating you up. That’s why you have a fever. Or perhaps you ate foods that were too warm or dry for your system. Maybe you were greedy at a feast and glutted yourself on spiced wine; perhaps you could not hold back from eating far too many roast birds.
Bad air might also imbalance your system. Air near the sea was cool and wet, while in low inland places it was warm and wet. We’ve often remarked from the standpoint of current knowledge of mosquito-borne illness that “bad air” most often had mosquitos whining about, although this connection was not made at the time. When people traveled to foreign places, they often came home sick, which was obviously caused by bad air or unfamiliar air that imbalanced a traveler’s system even if it was okay for the inhabitants.
The native Germanic idea of illness seemed to focus most on attacks. Your health was stolen, or something pierced your body and brought unwholeness to it.
In pre-Christian times, the attackers were often elves and dwarves. These creatures lived a life apart from mankind, in woodlands or in mountains and caves (respectively). They were not considered evil. They were just dangerous because they took no care for mankind or might be angered. Elves shot arrows, so sudden attacks of illness, especially if it appeared localized with swelling or a wound, were viewed as elf-shot attacks.
Another type of attack came from a worm. This included literal parasites, as well as imaginary evil serpents. But just as cancer is named for the microscopic crabs they thought were eating the flesh, other illnesses could be caused by invisible worms.
It would simplify things too much to reduce all illness to imbalance and outside attacks; there seems to have been also a general awareness that illness just happens. However, it was still a loss: a loss of balance or wholeness. Plants might restore what was lost, or charms might induce the loss to be reversed.
In early medical books, diseases and remedies were organized from head to foot. This suggests, too, that the illness was generally seen as localized. Disorders like diabetes and gout, which stem from internal organs but show up in limbs or eyes, were out of their reckoning. How much comprehension there was in the linkage of symptoms depends a lot on the place, time, and chain of influence among writers.