Current theory (structural dissociation) suggests there *isn't* a host at all - or what used to be thought of as a 'host' is now called an ANP (an 'apparently normal personality'). An ANP is any personality state who deals with everyday aspects of life. Current theory also suggests there can be more than one 'ANP', each who takes on a different job or aspect of a job. For instance one who takes care of the children and house, one who relates to a partner, another one who relates to a partner sexually, and one who deals with work or business. In this theory of dissociation it is quite normal to have more than one 'host'.
In structural dissociation it is believed that every single baby begins life with an unintegrated personality... meaning they have different emotional states that are simply reactive rather than regulated (ie: they cannot control their own emotional states). In normal development the child learns to regulate and control these emotional states with the aid of the parent. The parents model and teach them about healthy emotional boundaries, self soothing, and give them a sense of physical and emotional safety. When this doesn't happen the different emotional states remain unintegrated and separate, and if the child experiences trauma that cannot be emotionally resolved ongoing dissociation can develop.