Host (biology)

Lore & Background

The concept of a host spans a wide variety of relationships, differing in permanence and effects on the two parties. In parasitism, the parasite benefits at the host's expense; in commensalism, the two live together without harming each other; in mutualism, both parties benefit. Most parasites are only parasitic for part of their life cycle, and some organisms live in close association with a host and only become parasitic when environmental conditions deteriorate. A parasite may have a long-term relationship with its host, as with all endoparasites, while a parasitoid ultimately causes the host's death. The host range is the collection of hosts that an organism can use as a partner.

Reader's Guide

Hosts are fundamental to the study of symbiosis and disease. The type of host—definitive, secondary, paratenic, dead-end, or reservoir—determines the parasite's life cycle and transmission. For example, the definitive host is where the parasite reaches adulthood and reproduces sexually, while the intermediate host harbors the immature parasite and is required for development. Reservoir hosts harbor a pathogen without ill effects, serving as a source of infection to other species. In botany, host plants supply food to micropredators, and the host range of herbivorous insects can be monophagous, oligophagous, or polyphagous. Understanding host relationships is crucial for epidemiology, disease control, and conservation, as seen in the antigenic shifts of Influenza A virus when pigs act as hosts for multiple strains.

Did You Know?

• A parasitoid spends a large part of its life within or on a single host, ultimately causing the host's death.
• The heartworm of dogs uses the mosquito as its intermediate host until it matures into the infective L3 larval stage.
• Monophagous insects feed on a single plant, such as the silkworm larva which only eats mulberry leaves.