Parasitism

Parasitism is a type of symbiotic relationship in which one organism, the parasite, benefits by exploiting another organism, the host, at the host's expense. Parasites may be microscopic or visible, and may affect the host internally or on the surface. Parasitism includes microparasites (viruses, bacteria, protozoans) and macroparasites (worms, arthropods), and can occur inside the body (endoparasites) or on its surface (ectoparasites). Parasites can be obligate, requiring a host to complete their life cycle, or facultative, able to exploit a host but not dependent on it. They may be permanent or temporary, and they can be species-specific or able to infect a range of hosts.

Many parasites do not kill their hosts immediately, instead exploiting resources such as nutrients, tissues, or blood. In some systems, organisms called parasitoids kill and consume the host as part of their development; typical parasitoids are insects like certain wasps. Parasitism also includes parasitic plants that obtain nutrients from their hosts.

Life cycles often involve transmission between hosts, via direct contact, ingestion, or vectors such as mosquitoes or ticks. Some parasites are purely intracellular pathogens, while others live in tissues or the gut. They may manipulate host behavior or physiology to enhance transmission.

Hosts mount immune responses and behavioral defenses, while parasites evolve strategies to evade detection, proliferate, or alter host signals. The balance between virulence and transmission influences disease dynamics and population structure.

Parasitism is a major force in ecology and evolution, shaping species interactions, community structure, and biodiversity. It is also central to medicine, veterinary science, and agriculture, where control of parasites and their vectors reduces disease burden and economic loss. Research in parasitology aims to understand life cycles, host range, and mechanisms of pathogenicity.