Parasiitlus

Parasiitlus, or parasitism, is a non-mutual biological relationship in which a parasite benefits at the expense of its host. The parasite extracts resources, often causing harm or physiological changes that can aid its survival or transmission. Parasitism is one of several types of symbiosis and is distinct from mutualism, where both partners gain, and commensalism, where one party benefits without affecting the other.

Parasites are commonly classified by location (endoparasites living inside the host versus ectoparasites on the host’s surface), by size (microparasites such as protozoa and viruses versus macroparasites such as helminths and arthropods), and by life cycle requirements (obligate versus facultative; direct versus indirect with one or more hosts). Many parasites have complex life cycles that involve transmission between hosts and sometimes vectors, such as mosquitoes or ticks.

Life-cycle strategy and transmission influence virulence and host interactions. Parasites employ a range of adaptations to invade, evade the immune system, reproduce, and move between hosts. These interactions can shape host behavior, physiology, and population dynamics, and drive coevolutionary processes often described by Red Queen dynamics, where hosts and parasites continually adapt in response to each other.

Impact and control are context-dependent, affecting human health, agriculture, and wildlife. Parasitic infections contribute to morbidity and economic costs worldwide. Detection methods include microscopy, serology, and molecular assays. Control measures emphasize sanitation, vector management, targeted chemotherapy, and vaccines, though drug resistance and ecological considerations present ongoing challenges.