No one wants to come out of a lake or river covered in leeches and if your doctor pulls them out, you probably ought to run away as fast as you can...that is, unless you have arthritis. As it turns out, they can be particularly useful in those cases:
Slap four leeches on your knee and after 80 minutes, the pain and stiffness of osteoarthritis melts away. Of the 16 patients in the trial, the 10 who received leech therapy felt instant relief after application, and the comfort lasted for four weeks. The control patients continued experiencing pain. Researchers claim the leeches’ saliva works as an anti-inflammatory.
Learn about more pests-turned pros in this great Mental Floss article.