Parasiit

Parasiit is a biological term for an organism that lives on or in another organism (the host) from which it derives benefits at the host's expense. Parasiit relationships range from relatively benign to disease-causing, and the term covers a wide variety of organisms. The term parasiit derives from Greek parasitos, meaning "one who sits beside."

Parasiit classifications include endoparasites, which inhabit the interior of the host, and ectoparasites, which live on the surface. They can be microparasites, such as protozoa, bacteria, and viruses, or macroparasites, such as helminths (worms) and many arthropods. Life cycles may be direct, completed within a single host, or involve intermediate hosts and vectors.

Major groups include protozoa (for example Plasmodium, Giardia), helminths (nematodes such as Ascaris; cestodes such as Taenia; trematodes such as Schistosoma), and arthropods (ticks, fleas, lice).

Transmission occurs through ingestion of contaminated food or water, contact with contaminated surfaces, vectors such as mosquitoes or ticks, or direct vertical or sexual transmission, among others.

Parasites can cause a range of health outcomes from asymptomatic carriage to serious disease and systemic illness. They impose significant public health and economic burdens in many regions. Diagnosis relies on microscopy, serology, molecular methods, or imaging; treatment uses antiparasitic drugs, and prevention focuses on sanitation, vector control, protective behaviors, and vaccination where available.