An organism that supports the survival, growth or reproduction of another organism during part or all of its life cycle.
Explanation
In microbiology, the term host refers to a living organism that furnishes resources and habitat for another organism, such as a virus, bacterium, protozoan, helminth or fungus. Host relationships underpin parasitism, commensalism and mutualism. A definitive host is one in which a parasite reaches sexual maturity or completes its final stage of development. Intermediate hosts harbour immature or larval stages and are required for transmission. Some parasites also use transport or paratenic hosts that are not essential for development but assist movement between definitive hosts. Reservoir hosts maintain pathogens in nature and can seed infections in susceptible species, whereas incidental hosts are not normally part of the life cycle but may become infected. Host specificity depends on factors such as cell surface receptors, immune defences and environmental niches. Adaptive responses by hosts, including innate and adaptive immunity, shape host–microbe interactions and influence pathogen evolution. At a cellular level, viruses exploit host machinery for replication, and some bacteria become endosymbionts within eukaryotic cells. Understanding host roles is critical for disease ecology, vaccine design and control of zoonotic diseases.
Examples of Host Relationships
Humans are definitive hosts for Plasmodium falciparum, the protozoan that causes malaria, while Anopheles mosquitoes serve as intermediate hosts transmitting the parasite. Freshwater snails act as intermediate hosts for Schistosoma trematodes, and mammals become definitive hosts when they contact contaminated water. Dogs and wild canids are reservoirs for rabies virus, which can spill over to humans through bites. In plant pathology, crown gall disease results from Agrobacterium tumefaciens transferring DNA into susceptible plants. Bacteria themselves can be hosts for bacteriophages, viruses that infect and replicate within bacterial cells. These examples illustrate diverse host–microbe associations across kingdoms.
The host concept highlights the dynamic interplay between organisms. It informs studies of pathogenesis, symbiosis and ecological networks by focusing on who supports whom. Recognising host categories aids the control of infectious diseases and helps explain why certain species are susceptible to particular pathogens.
Related Terms: Parasite, Pathogen, Reservoir, Symbiosis, Definitive host