Parasitism is a non-mutual symbiotic relationship between species, where one species, the parasite, benefits at the expense of the other, the host.

Unlike predators, parasites do not kill their host, are generally much smaller than their host, and will often live in or on their host for an extended period.

Classic examples of parasitism include interactions between vertebrate hosts and tapeworms, flukes, the Plasmodium species, and fleas.

Photographer Marcus DeSieno‘s photo series Parasites for lots more from the series.
Quoted text via Wikipedia.