It is a very, very, tiny little bug. Unlike ticks, which cling to the outside of your skin when they bite, the chigger burrows inside and begins feeding. It leaves a little "snorkel," finer than a hair, outside your skin to breathe through. When you put chigger medicine, or simply clear nail polish, over the bite it eventually suffocates the little bugger. I've also heard of people using pure Clorox to kill them, though I suspect that is more painful than the bites!

John