A host body was a body used, infected or possessed by another creature, usually one who was incorporeal or very small. In some cases the possession was fatal to the host.
The Gestalt bonded over a succession of willing hosts over its long existence. In exchange for the host receiving the Gestalt's advice and increased physical durability, they would be required to follow the Gestalt's direction to fulfil the Gestalt's mission of righting wrongs across the universe. (PROSE: Ring Theory)
Lady Cassandra used several bodies as hosts, including Rose Tyler, the Tenth Doctor and Chip when she left her own body, through the use of a psychograft. (TV: New Earth)
Historians who had been allowed access to the "Monster Vaults" of the databanks in the Doctor's TARDIS observed that a victim in the process of being absorbed by an Abzorbaloff would become mentally linked with their host, allowing them to read their minds and thoughts. (PROSE: The Monster Vault, TV: Love & Monsters)
In a dream, Eknodines used many bodies as hosts, including Poggit, Hamill and Nainby. (TV: Amy's Choice)
The Isolus child used Chloe Webber as a host body. (TV: Fear Her)
The Gorgon used the Abbess as a host body for a long time and chose Sarah Jane Smith as a replacement. (TV: Eye of the Gorgon)
The Sex Gas possessed the body of Carys Fletcher. (TV: Day One)
Judson and Sorin acted as the host bodies for Fenric. (TV: The Curse of Fenric)
Toby Zed and various Ood were possessed by the Beast. (TV: The Impossible Planet / The Satan Pit)
Sarah Jane Smith was possessed by an Eight Legs. (TV: Planet of the Spiders)
The Seventh Doctor was temporarily possessed by the Timewyrm. (PROSE: Timewyrm: Revelation)
The Gelth took many corpses in Gabriel Sneed's mortuary as host bodies, including Mr Redpath, his grandmother, Mrs Peace and Sneed himself. (TV: The Unquiet Dead)
The Host was a human boy who was infected by a lupine wavelength haemovariform that had survived a crash landing on Earth by passing into the bloodstream of several hosts and adapting through the generations. (TV: Tooth and Claw)
The High Priestess of the Sibylline Sisterhood was a host body for a Pyrovile in an early stage. (TV: The Fires of Pompeii)
Sky Silvestry and the Tenth Doctor were possessed by the Midnight entity (TV: Midnight)
The Mayfly needed host bodies as they grew. They needed a perfect host and so cleared the body of all imperfections. (TV: Reset)
The Nostrovites needed a host for their eggs as a third member of their reproduction process. (TV: Something Borrowed)
Meglos used an "earthling" as a host body to impersonate the Fourth Doctor. (TV: Meglos)
Androvax possessed the body of Sarah Jane Smith. (TV: Prisoner of the Judoon)
Kroagnon possessed the body of the Chief Caretaker to escape his imprisonment in the basement of Paradise Towers. (TV: Paradise Towers)
Wirrn gestated inside host bodies and absorbed the intellect and memories of their hosts. (TV: The Ark in Space, AUDIO: Wirrn Dawn)
The Fourth Doctor and several crew members of the Bi-Al Foundation and Titan Base were hosts for the Swarm led by the Nucleus. (TV: The Invisible Enemy)
Four townspeople were killed and had their bodies taken over by the Family of Blood. (TV: Human Nature / The Family of Blood)
The Torajii sun used number of host bodies, including the Tenth Doctor, to destroy the ship that had drained its power. (TV: 42)