Firstly, I'd like to make a fairly important distinction: the king's touch wasn't supposed to cure what we now commonly call tuberculosis (i.e. pulmonary TB) but scrofula. Scrofula is mainly caused by the same bacterium, and while it is a visually impressive disease (it infects lymph nodes and causes large growths in the neck), it is not as contagious as pulmonary infections (if somebody with more medical knowledge could confirm that, it'd be ace), although the two can of course go together. The medieval prognostic for sufferers of scrofula also seems to have been a bit better than that of people affected with pulmonary TB, but probably not by a great deal. Since the discovery of micro-organisms was still several centuries away, what was termed "the King's Evil" probably included more benign diseases with symptoms resembling those of scrofula proper, such as goitre and mumps.

I mostly know the history of the practice in France (a good but dated reference is Pierre Bloch's doctoral thesis, Les Rois thaumaturges/The Royal Touch, a fairly important text in the development of modern historiography in its own right), but it seems in later centuries contamination had become a real concern, despite the fact that its precise mechanism was obviously not understood. The official French ceremonial for the royal touch in the 17^th century included a "maître d'hôtel [holding] a towel soaked with wine and water to give to the King and wash his hand after so many dirty touches" (my shitty translation). Several contemporary documents also attest that surgeons examined the patients beforehand, eliminating those who didn't have scrofula (the touch came with a small coin as alms, which provided an incentive beyond the king's purported powers). Presumably the more suppurating patients would have been turned away at this stage. By the way, Louis XIII, who followed this ceremonial, did die of tuberculosis, but there's little to suggest that it was a direct consequence of the king's touch.

The specifics of older practices are less documented, and records indicate that ceremonies involved several hundred or a even a thousand sufferers at a time, which does beg the question of how kings avoided contamination. Bloch hypothesises that a great deal of the people the king touched didn't actually have scrofula or "écrouelles", but less severe illnesses.

Interestingly, in contrast to the French tradition, English kings began in the 15^th century to touch coins that were then to be distributed to the population as cure or protection against not only scrofula, but epilepsy, joint pain, etc. a practice that was obviously less dangerous and more pleasant for the monarch--although the actual laying of hands continued in parallel.