The most common concept of medieval medicine that I hear is “bloodletting to release the evil spirits.” Though I have found recommendations that people with certain dispositions undergo bloodletting at certain times of year, I have never found a reference to “evil spirits.” Bloodletting practices are related to the idea that just as the heavens affect seasons and tides on Earth, they also affect human bodies. Man was a microcosm composed, like the earth, of 4 elements that had to be balanced.  However, for a specific illness, bloodletting usually required the following symptoms:
- When let, the blood is thick and black or dark-colored 
Books usually state that if the blood is bright when it flows, then the physician should stop immediately because bloodletting will be of no benefit. Medieval doctors considered bloodletting a serious ordeal since it caused weakness and great chances for infection, so many places established strict laws regarding bloodletting.  In reading medieval literature, I have yet to find a reference to bloodletting (this isn’t an assertion that they’re not there—I just haven’t found any, so they’re not terribly common). In Arthurian literature, where the usual injuries are battle wounds with occasional poisoning, the remedy is almost always either bandages and rest or a miraculous ointment.
A stereotype exists that the Church forbade dissection in the Middle Ages, which led to stagnation in surgery. Surgery made great advances during the Middle Ages with many extremely skilled practitioners.  The decree of Pope Boniface VIII about dissection only prohibited boiling of corpses on the battlefield so that the clean bones could be sent home. It did not prevent dissections for medical research, for plenty of records exist of such dissections.  One greatly influential book was a surgery book by Roger of Salerno, which includes a number of medical areas, not just surgery, but dealt extensively with surgery for skull injuries.  Salerno focuses largely on wounds from weapons, but also mentions the possibility of surgical treatment for uterine cancer. 
Experience and education were both considered vital for medieval surgeons. The medical school at Montpelier in the 13th century required students to observe surgeries on “the head, intestines, and other internal organs,” noting that nobody should attempt an operation before he had observed it.  From Montpelier, we have records of surgery for cataracts, liver injuries, ruptured peritoneum, dislocation of neck vertebrae, and hernia.  From Iceland, a study of skeletons from the period shows numerous treatments for sword injuries to the head that were treated by trephining, and in many cases, bone growth shows that death came far after the injury had healed, proving the surgeons’ skills.  Medieval medical books show that most surgeons considered a punctured lung untreatable, but William of Saliceto claims to have cured a man of that injury. 
An idea seems to be floating around that all surgery was performed by barbers. I’m having a difficult time in ascertaining how common this practice was. In Medicine in Medieval England, Talbot’s chapter on surgery deals entirely with physicians and surgeons as specifically medical men, not as multitasking hairdressers, which leads me to the assumption that barber-surgeons were uncommon in areas where medical schools were accessible. I am led to believe that bloodletting, considered surgery, was far more likely to be performed by a barber-surgeon than the treatment of serious injuries would be. [Check the comments for some excellent resources on barber-surgeons.]
Illnesses in General
Medical books are overwhelmingly filled with advice to eat a diet properly balanced in meat and vegetables, to get an adequate amount of exercise and rest, and to maintain proper hygiene. Other medical treatments were largely herbal. Some were dangerous, some were certainly fatal, but most probably worked. After all, if a practice never brought results, it would not have been practiced for long.
When reading medieval medical books, one must keep two things in mind. First, do not try the remedies. That should be obvious. Second, remember that every person who wrote about medicine may not have been a doctor, and just because something is written does not necessarily mean that it was widely practiced. H.M. Cholmeley dedicates a section of his studies to an evaluation of the medieval quack doctor. He gives an example of a typical speech proclaimed to lure in unwitting customers with promises of miraculous cures:
Good people! I am not one of the poor preachers, nor one of those poor herbalists who go in front of churches with their shabby ill-sewn cloaks, who carry boxes and bags and spread out (their wares) on a carpet, for they sell pepper and cumin and other spices. Know that I am not one of them, I belong to a lady whose name is Trote (ie. Trotula) of Salerno…and know that she is the wisest lady in all the four quarters of the world. And my lady sends us out…. 
Unfortunately, much of my information on herbal remedies comes from .pdf fragments that I received from professors in graduate school and cannot cite. Many are likely reading this post for an overview of the ridiculous practices that medieval man mistook for medical treatment. I’m afraid that you are likely to be disappointed. Most medieval medical remedies are quite mundane. Effectiveness varied, but most of it was reasonable, showing that before modern medicine, man made continuous advances in caring for those in need.
- Artz, Frederick B. The Mind of the Middle Ages: An Historical Survey A.D. 200–1500, 3rd ed. (1953; repr., Chicago, University of Chicago Press: 1980), 127.
- Ibid., 131.
- Ibid., 128.
- Artz, The Mind of the Middle Ages, 244.
- Cholmeley, H.M., John of Gaddesden and the Rosa Medicinae (Oxford, Clarendon: 1912), 107.
- Talbot, C.H. Medicine in Medieval England (London: Oldbourne, 1967), 88-89.
- Ibid., 88-90.
- Ibid., 93.
- Ibid., 94.
- Ibid., 96-97.
- Ibid., 100.
- Cholmeley, John of Gaddesden, 88-89.