Of more than 1200 described species of trombiculid mite, only about 50 are known to attack humans or livestock. Only the larvae, popularly known as chiggers, are parasitic on vertebrates. They are widely distributed and in many countries, cause a dermatitis in humans, although not all chiggers cause an itchy reaction.

Trombiculid larvae (Figures IV.4, IV.5) normally parasitize rodents and birds but, given the opportunity, will feed readily on humans. The non-parasitic female lays eggs in damp but well drained soil. Typical breeding places are cultivated alluvial river banks in Japan, scrub jungle, grassy fields or untended gardens with a rank growth of grass and other vegetation.