They are the larval stage of mites which belong to the family Trombiculidae. Mites belong to the same group of organisms as ticks, the Acari. Chiggers will readily feed on reptiles, amphibians, small mammals, and birds which are their normal hosts.
People are accidental hosts.
Chiggers that attach to humans are usually dislodged or die within hours. The human immune reaction to a bite prevents the chigger from obtaining adequate nourishment. They rarely survive long enough to complete a meal. Chiggers do not burrow into the skin or suck blood. Rather, they suck tissue fluids with their mouthparts.