Parasite

A parasite is an organism that lives on or in another organism, the host, from which it derives nutrients and other benefits. Parasites can harm the host by consuming resources, eliciting immune responses, or causing tissue damage. The parasitic lifestyle contrasts with mutualism and commensalism, where interactions do not harm or even benefit the host. Parasites may rely on a single host or require multiple hosts to complete their life cycle.

Parasites are commonly categorized by their location as endoparasites, which inhabit internal tissues or body cavities, and ectoparasites, which feed on the surface or external environment of the host. They can also be classified by their dependency and life cycles, such as obligate parasites that cannot complete their life cycle without a host, and facultative parasites that can live independently but may exploit hosts opportunistically. Some parasites use vectors to move between hosts, a strategy that can influence transmission dynamics.

Many parasite groups affect humans and other animals. Protozoan parasites include Plasmodium (causing malaria) and Giardia. Helminth parasites encompass nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes). Ectoparasites such as lice, fleas, ticks, and mites live on the skin or outer surfaces. Transmission routes include contaminated food or water, insect vectors, direct contact, and skin penetration.

Parasites often have complex life cycles involving one or more hosts, sometimes including intermediate vectors. Infections may be acute or chronic, and disease results from tissue damage, nutrient depletion, and immune responses. Heavy parasite burdens can cause significant morbidity and, in some cases, mortality.

Prevention and control focus on hygiene, sanitation, vector control, safe food and water, and protection against vectors. Treatments employ antiparasitic drugs, which may target specific life stages. Public health measures and veterinary controls are essential to reduce transmission and economic impact in agriculture and wildlife.