Hosts are creatures which can serve as survival methods for symbiotes and parasites. The Goa'uld and Tok'ra both require hosts to survive, but their methods of acquiring hosts are extreme opposites. The Goa'uld forcibly steal hosts from their homeworlds, taking complete control of their bodies and forcing them into what is described as a living hell. In sharp contrast, the Tok'ra do not take hosts, but rather acquire their consent before entering. The Goa'uld insist that nothing of the host survives, but this has been dis-proven on a number of occasions.
While in a host, a Goa'uld and Tok'ra can increase the host's intelligence and strength, and give it a near perfect immune system. They also secrete Naquadah into their host's blood stream (though wild Goa'uld on P3X-888 do not do this). The symbiotes can also make their host's eyes glow at will, and usually speak in a deep, booming voice. Known hosts who no longer have a Goa'uld symbiote in them include Vala Mal Doran, who used to host to a Goa'uld Underlord named Qetesh but was freed by the Tok'ra; Colonel Steven Caldwell who was the host to an unnamed Trust operative;and Captain Samantha Carter who was the unwilling host to a Tok'ra called Jolinar of Malkshur who sacrificed herself to save Carter. Also, there was Kendra before she was killed and the host to the last remaining Ba'al clone who had his symbiote extracted from him and killed, leaving him alive and free. All have the ability to use all Goa'uld technology due to Naquadah in their blood from the blending and can also sense other symbiotes. (SG1: "Thor's Hammer", "In the Line of Duty", "Thor's Chariot", "The First Ones") (ATL: "Critical Mass")
It has also been shown that a strong host can overpower the symbiote at times. However, the symbiote will eventually regain control. Skaara has shown this ability to overpower Klorel three times, once to influence Klorel to let SG-1 live against Apophis' wishes. The second was an attempt to stop Klorel from raising the shields of the Ha'tak so it would be destroyed, but Klorel regained control and raised the shields. The third time was with the aid of a Tollan Detachment device. This method, however, allowed both host and symbiote complete control and prevented one from impersonating the other. (SG1: "The Serpent's Lair", "Within the Serpent's Grasp", "Pretense")