Host (biology)

In biology and medicine, a host is a larger organism that carries a smaller organism. This smaller guest is known as a symbiont. The relationship between them can vary greatly depending on how they affect one another. Most often, the host provides the guest with necessary nourishment and shelter. These biological connections are central to how life functions, from tiny viruses to large animals.

Symbiosis describes the various ways these organisms live together. These relationships are categorized by how they impact the two parties involved. In mutualism, both the host and the guest benefit from the association. Commensalism occurs when one partner benefits while the other is neither helped nor harmed. Parasitism is different because the parasite benefits at the direct expense of the host. Some guests, called parasitoids, live within or on a host and eventually cause its death. This is a more extreme strategy than typical parasitism, which aims to keep the host alive.

Scientists classify hosts based on the specific role they play in a parasite's life cycle. A definitive or primary host is where a parasite reaches its adult stage and reproduces sexually. An intermediate or secondary host is required for the parasite to develop and complete its cycle. For example, the heartworm of dogs uses a mosquito as an intermediate host until it reaches its infective L3 larval stage. Some organisms act as paratenic or transport hosts. These hosts carry sexually immature parasites that do not undergo further development there. They essentially serve as storage sites where parasites can accumulate before being eaten by a definitive host.

Other types of hosts are defined by how they impact the spread of a disease. A reservoir host harbors a pathogen but suffers no ill effects from it. These animals can serve as a constant source of infection for other susceptible species. In contrast, a dead-end or accidental host does not allow the parasite to complete its development. Humans and horses are dead-end hosts for the West Nile virus. While they can become infected, the virus level in their blood is not high enough to pass it back to mosquitoes. This effectively stops the transmission cycle in those specific individuals.

In the world of botany, hosts interact with small predators known as micropredators. These are often herbivorous insects that eat parts of a host plant. An insect's host range is the collection of different plants it can use as a partner. Some insects are monophagous, meaning they feed on only one specific plant type. The silkworm larva is monophagous because it eats only mulberry leaves. Other insects are oligophagous, meaning they are restricted to a few closely related species. Finally, polyphagous insects, like the buff ermine moth, can feed on a wide variety of different plant families.

Host relationships can also be entirely beneficial, creating complex ecosystems. Many marine invertebrates, such as corals, host single-celled algae called zooxanthellae in their tissues. The host provides a protected, well-lit environment, while the algae provide nutrients through photosynthesis. In the soil, over 95% of plant families participate in mycorrhizal associations. This is a mutualistic link where a fungus connects to plant roots. The plant provides carbohydrates from photosynthesis, and the fungus supplies essential phosphates and nitrogen.

Understanding host ranges is also critical for managing global health. The host range of a virus can influence how a disease spreads through a population. For instance, the Influenza A virus can undergo antigenic shift. This happens when a pig is infected by viruses from different hosts, such as humans and birds. The different viral genes can mix, creating a brand new strain. This process can make existing vaccines ineffective, requiring scientists to develop new ones to protect people.