“Leeches are able to display a variety of behaviors that allow them to explore their environments and feed on their hosts.” – Wikipedia
You know, when you go for a swim in a lake, it’s all good and well.
And when you go out of the water, you see that your body is covered with leeches.
If you let them stay too long, they’ll drain you.
Get rid of them as fast as you can. Even though it might hurt at first.