A bloodsucker is a creature that feeds on blood from other animals. Leeches, mosquitoes, ticks, and vampire bats are all bloodsuckers. They attach themselves to a host (the animal they feed from) and draw out blood for nourishment. Some bloodsuckers, like mosquitoes, land quickly, bite, and fly away. Others, like ticks, latch on and feed for hours or even days.

While bloodsuckers sound creepy, many serve important roles in nature. Some birds and fish depend on mosquitoes for food, and doctors once used leeches in medicine (and still do occasionally today). However, bloodsuckers can also spread diseases when they move from one host to another.

People also use bloodsucker as an insult for someone who takes advantage of others, especially by constantly asking for money or favors without giving anything back. If someone always borrows lunch money but never pays it back, their friends might joke that they're a bloodsucker. Like the creature, this kind of person drains resources from others while contributing nothing in return.