As usual, if a question can't be answered, it deviates to god/goddesses dayon.
And to answer your question(opinion):
Scientists/Doctors used leeches to cure diseases.
Someone/Something used humans to accumulate knowledge for their own use. Humans are unique creatures, it accumulates knowledge over time. If we discover something, then the one using us is very happy. In short, we are guinea pigs.
"Leeches sucks blood, humans sucks knowledge"