The larva attaches where skin is thin or clothing is tight, pierces a hair follicle or pore, and injects digestive enzymes. Host cells around the puncture harden into a tube called a stylostome, which the mite uses as a straw. The itching, which typically peaks a day or two after attachment, is a reaction to the enzymes and the stylostome rather than to any wound the mite makes.
Because it is anchored to the stylostome, a chigger stays in place for days if undisturbed, though most are scratched off long before feeding is complete. It never enters the skin, does not lay eggs in the host, and does not need to be smothered with nail varnish; the folk remedies work only in the sense that the mite is usually gone already.
Most trombiculid chiggers transmit nothing. The important exception is in Asia and the western Pacific, where certain species transmit Orientia tsutsugamushi, the agent of scrub typhus. Which species are competent vectors, and how widely scrub typhus actually occurs, are still being revised.