Final answer:
A parasite is an organism that lives in or on another living organism, deriving nutrients from it. It benefits from this relationship, while the host is harmed. Parasites usually weaken their hosts by siphoning resources and have adaptations to prolong the host's survival for completion of their reproductive cycle.
Step-by-step explanation:
A parasite is an organism that lives in or on another living organism and derives nutrients from it. In this relationship, the parasite benefits, but the host is harmed. The host is usually weakened by the parasite as it siphons resources the host would normally use to maintain itself. Parasites may kill their hosts, but there is usually selection to slow down this process to allow the parasite time to complete its reproductive cycle before it or its offspring are able to spread to another host. Parasitism is a form of predation.