Leeches were so valuable that some people worked collecting them
This may not be news but during the 19th century, doctors used to believe that leeches could cure any kind of affection or disease by sucking it out of the body. We now know that this bit of medical advice is completely false. However, back then, they did not have enough information to prove that this theory was wrong.
Therefore, it was a popular medical method. In fact, there was such a demand for leeches that there were people who were dedicated to catching leeches and selling them to doctors. The collectors’ business must have been pretty profitable since doctors used more than a few leeches to treat every one of their patients.