A parasite is anything that takes and does not give back. Some parasites are large, for example a cuckoo laying its eggs in the nest of another bird, while some, like viruses are small. The picture shows a tapeworm in a human gut. Fleas, mites and head lice are more examples that affect people.

Parasites do not kill their host, and will often live in, on or beside their host for quite a while. For this time they get food, water, heat, a nice place to live, and the chance to move on to another suitable host when they are ready.

Sometimes the result of parasites can be catastrophic, although that is not what the parasite had in mind. The plagues including the Black Death were a result of parasites.