What do we call an organism that lives on a host which it usually injures?

The term for this type of organism is derived from the Greek "parasitos," which translates to "one who eats at another's table." This paints a vivid picture of the relationship: an organism that lives on or inside another, known as the host, and benefits by deriving nutrients at the host's expense. Unlike other symbiotic relationships, this one is defined by the harm it causes. The parasite (Review) gains food, shelter, and a place to reproduce, while the host is weakened, injured, or suffers from disease as a direct result of the interaction.

This one-sided arrangement stands in contrast to other forms of symbiosis. In mutualism, both organisms benefit, such as a bee gathering nectar while pollinating a flower. In commensalism, one partner benefits while the other is largely unaffected, like a barnacle riding on a whale. Parasitism is distinct because the host's well-being is always compromised for the parasite's survival and propagation.

These organisms are incredibly diverse and widespread in the natural world. They range from microscopic protozoa, like the Plasmodium that causes malaria, to much larger creatures like tapeworms living inside an animal's gut or ticks and fleas that live on the outside. This exploitative lifestyle has proven to be an enormously successful evolutionary strategy, with parasitic species existing in nearly every branch of the tree of life.