A parasite is an organism that depends on another living host for nutrients or shelter and usually causes some degree of harm to the host in the process.
Explanation
Parasites have evolved across many branches of life, from single-celled protozoa to multicellular worms and arthropods. They survive by exploiting a host for resources while offering no benefit to that host. Endoparasites live within the host’s body; examples include intestinal helminths such as tapeworms and roundworms, and protozoa like Plasmodium species that inhabit blood cells. Ectoparasites live on the surface of their hosts and include insects and arachnids such as mosquitoes, ticks and lice that feed on blood. Some parasites require only one host to complete their life cycle, while others rely on multiple hosts or vectors to move from one stage to another. Many display complex adaptations, such as hooks or suckers for attachment, and mechanisms to evade or modulate the host’s immune response. Parasites can be highly host-specific, infecting only one species, or generalists that infect many. Their presence can impair host growth, reproduction or survival, and in human populations parasitic diseases impose a significant global health burden. Studying parasites provides insights into evolutionary biology, immunology and ecology.
Representative Examples
Plasmodium falciparum is a protozoan parasite transmitted by Anopheles mosquitoes; it invades human liver and red blood cells, leading to malaria. The nematode Ascaris lumbricoides lives in the human intestine, causing abdominal discomfort and malnutrition. Tapeworms of the genus Taenia attach to the intestinal wall of mammals and can reach several meters in length. External parasites include the flea Ctenocephalides felis, which feeds on the blood of cats and dogs, and the deer tick Ixodes scapularis, which transmits Lyme disease. In the plant kingdom, the dodder vine (Cuscuta species) wraps itself around host plants to withdraw water and nutrients. These cases illustrate the diversity of parasitic strategies and the wide range of hosts they exploit.
Parasites are a diverse group of organisms unified by their reliance on hosts for survival. Their ability to manipulate and exploit host biology makes them both fascinating subjects of study and significant agents of disease.
Related Terms: Parasitic, Parasitism, Host, Endoparasite, Helminth