The term originated as a corruption of the name of the chigoe flea{Sarcopsylla, or Pulex, penetrans}, a tropical flea common in South America and the West Indies. The breeding females burrow into exposed skin and lay eggs, causing strong irritation and itching. If left inside the skin this may lead to infection or other dangerous complications.
The term is also used to refer to larvae of certain mites which bury themselves in exposed skin in order to feed on the blood of their host. Unlike the chigoe flea, these mites may be either male or female, and do not lay eggs inside their host. This usage, especially common in the United States, likely originated as a misidentification of the mites as chigoes, due to their similar burrowing behavior. However, it has become widespread enough that it is now generally considered a correct usage of the term.
Chiggers (aka Red Bugs, Harvest Mites) are chrome-orange mites that live in berry patches, tall grass and weeds, woodland edges, pine straw, leaves, and treebark. These relatives of spiders are nearly microscopic measuring 1/100 of an inch. The chigger's 6 legged parasitic larva will crawl onto humans, rabbits, toads, box turtles, quail, and even some insects, and bite the host. This causes severe itching, red pimple-like bumps (papules) or hives and skin rash or lesion on a sun-exposed area. Sometimes the larva is visible inside the center of the lesions. For humans, itching usually occurs several hours after the larvae attach to the skin. After 1-7 days the larvae drop off their hosts and mature into adults, which have 8 legs and do not harm humans. The females lay 3-8 eggs in a litter, usually on a leaf or under the roots of a plant, and die by Autumn.