Leeches are not harmful to humans. In fact, they are quite helpful. Leeches do not draw enough blood from a human to cause any damage and have been used for thousands of years in the medical field for a variety of reasons, including their ability to help reattach body parts.
Leeches produce an anti-coagulation agent that is transferred through their saliva. After a leech bites a human, this agent keeps the site of the bite from coagulating. The lack of coagulation allows the site to bleed out for hours after the leech has been removed. This allows oxygen to enter the blood through the wound, promoting vein re-growth and blood circulation. Leeches are invaluable in grafting and reattachment surgeries, which often involve tiny veins that are unable to be reattached by hand. The veins are kept from coagulating by attaching leeches to a grafting or attachment site, allowing existing veins to fuse and new veins to generate.
Leeches produce a natural anesthetic that allows them to bite without causing pain. While leeches on the surface of the skin are harmless and even beneficial, they should never be ingested. Humans should be cautious around them and ensure that they do not invade the cavities of the body, like the nose.