“Medieval doctors bled their patients with leeches. Far from improving their condition, it left them worse off.”
Leeches are starting to be used again in modern medicine.
I did a little googling and found this:
http://www.leechestherapy.com/history_of_leeches.php
“Recently, however, leeches have once again found their place in the field of medicine. They are now being bred in captivity in several institutions and leech farms are once again being created, but the use of leeches has become quite sophisticated. They’re not just used for anything, they are also used in the field of microsurgery and to release or drain the congested blood in some wound sites and are preferred in this area because of their precision. Plastic surgeons also use leeches to treat challenging skin grafts and most especially for reconstructive surgery.”
Regarding another medical practice that made a comeback big time, is using staples instead of stitches.
The Roman military used them for battle wounds. Made them out of silver, which has anti-bacterial properties, and were pre-scored to create weak points making them easier to remove.
Leeches are particularly useful in places where it is hard to dress the injury. For example within an old wound cavity.