A parasite is a living thing that survives by taking what it needs from another living thing, called the host, without giving anything back. The parasite benefits while the host suffers. Fleas are parasites that live on dogs and cats, biting them to drink their blood. Tapeworms are parasites that live inside animals' intestines, stealing nutrients from the food their host eats. Mistletoe is a plant parasite that grows on trees, taking water and minerals while weakening the tree.

Parasites have evolved clever ways to survive: ticks latch onto skin with hooked mouthparts, and some parasitic wasps even lay eggs inside other insects. While the relationship might sound simple, it's one-sided and harmful. The host gets weaker, sicker, or uncomfortable, while the parasite thrives.

People also use parasite as an insult for someone who takes advantage of others' generosity without contributing anything themselves. If someone constantly borrows money, eats your lunch, or copies your homework without ever helping you in return, they're acting like a parasite. The word carries a harsh judgment because parasites don't just receive help, they drain their hosts while giving nothing back.