Parasites

Parasites are organisms that live on or inside another organism, the host, and obtain benefits at the host's expense. They may inhabit the exterior surface or reside within tissues and organs, often relying on the host for nutrients, shelter, or dispersal. Parasites that live on the body surface are called ectoparasites, while those that inhabit internal organs or cells are endoparasites.

Parasites span a range of life forms, including protozoa (single‑celled organisms such as Plasmodium and Giardia), helminths (multicellular worms such as nematodes, cestodes, and trematodes), and arthropods (insects and arachnids such as ticks, fleas, lice, and mites).

Many parasites have complex life cycles that involve one or more hosts, vectors, and developmental stages. Transmission can occur through contaminated food or water, direct contact, or via vectors such as mosquitoes or ticks. The impact on hosts ranges from asymptomatic carriage to severe disease, malnutrition, tissue damage, or impaired development. Parasites and hosts often engage in evolutionary arms races, shaping each other's biology.

Diagnosis relies on clinical signs supplemented by laboratory methods such as microscopy, serology, or molecular tests. Treatments use antiparasitic drugs targeted to specific groups; prevention emphasizes sanitation, vector control, safe food handling, and protective measures to limit exposure. Public health programs monitor parasite distribution and implement vaccination when available or feasible.