Parasitism is a relationship where one organism, called a parasite, lives on or inside another organism, called a host, and survives by taking resources from it while harming it in the process. Think of a tick feeding on a dog's blood, or tapeworms living in an animal's intestines and stealing nutrients from its food. The parasite benefits while the host suffers.

This relationship is different from other ways organisms interact. In mutualism, both creatures benefit, like bees getting nectar while pollinating flowers. In commensalism, one benefits while the other isn't really affected, like birds nesting in trees. But in parasitism, the relationship is one-sided: the parasite takes what it needs, and the host gets nothing but trouble in return.

Parasites have evolved remarkable strategies to survive. Some manipulate their host's behavior to help them spread. Others can live undetected for years. While parasites rarely kill their hosts immediately (after all, they need them alive to keep feeding), they can weaken them significantly, making hosts more vulnerable to disease or predators.

Scientists study parasitism to understand ecosystems and develop treatments for parasitic diseases. The word is also used metaphorically to describe people who take advantage of others' generosity without giving anything back, though this usage can be insulting.