A group of vampire scientists implanted specially designed molecules in all humans' brains to separate healthy blood from the ones who have a sickness, like cancer, AIDS, diabetes, etc.

The ones with healthy blood have red nails. The ones with sick blood have brown nails.
Many healthy humans have tried to change their red color to brown using polish and other chemicals, and failed.
The color disappears within seconds of applying it.

People with red nails are forced to give blood three times a month. We have clinics that perform that kind of task all over the world.
If a human skips four visits he or she is hunted and terminated immediately.