A parasite is an organism that lives at the expense of another organism - the host.
In general, parasites share the following features:
• Parasites are usually smaller than their host.
• Parasites use both invertebrate and vertebrate hosts.
• Adult parasites may live on the host (e.g. lice), in the host (e.g. tapeworms) or feed on a host occasionally (e.g. mosquitoes).
• Parasites generally do not kill the host but may harm the host indirectly by spreading pathogens. This may affect the host's behaviour, metabolism or its reproductive activity.
• Many parasites have hooks, claws or suckers to attach to their host.
• Generally parasites have either a sucker (e.g. leeches) or piercing and sucking type mouthparts (e.g. fleas) for feeding.
• Both adults and young can be parasitic. In some cases the young are parasites but the adult is not.