Parasitism is a biological relationship in which one organism, the parasite, lives on or within another organism, the host, from which it obtains nutrients or shelter while causing harm to the host.
Explanation
Parasitism is one of the major forms of symbiosis, distinct from mutualism and commensalism. Parasites exploit their hosts for resources such as nutrients and space, often reducing host fitness through tissue damage, nutrient loss or immune-mediated pathology. Parasites range in size and complexity from viruses and bacteria to protozoa, helminths and arthropods. Microparasites, including viruses and protozoa, multiply within the host, whereas macroparasites such as worms and ectoparasitic insects typically do not multiply to high numbers inside a single host. Many parasites have complex life cycles involving multiple hosts or stages; for example, trematodes cycle between molluscan and vertebrate hosts. Adaptations to parasitic life include specialized attachment organs, evasion of host immunity and high reproductive output. Hosts respond with immune defenses, leading to coevolutionary dynamics between parasite virulence and host resistance.
Examples and ecological impact
Human diseases such as malaria, caused by Plasmodium species transmitted by Anopheles mosquitoes, illustrate parasitism at the microscopic level. Helminths like tapeworms, hookworms and schistosomes inhabit the intestinal tract or blood vessels and absorb nutrients directly from the host. Ectoparasites such as ticks, lice and fleas feed on blood and can transmit additional pathogens. Plant parasites include mistletoes and nematodes that invade roots and tissues. Parasites influence population dynamics, community structure and energy flow by regulating host abundance and behavior. Control strategies range from improving sanitation and vector control to anthelmintic drugs and vaccination.
Parasitism underscores the interconnectedness of organisms and the evolutionary arms race between exploiting and resisting. Understanding parasitic relationships informs medicine, agriculture and ecology.
Related Terms: Symbiosis, Commensalism, Mutualism, Host, Parasite