Chiggers
Chiggers are tiny, reddish orange arachnids with translucent legs. They measure only 1 millimeter in diameter, making them nearly invisible to the naked eye. Especially common in warm, humid climates, only their larvae bite humans. They feed on a variety of animals, but can't gain nutrients from biting humans so they die shortly after burrowing into human skin. They are accomplished jumpers, and frequently live in mossy trees and jump down onto people.
Mites
Several species of mites are tiny enough to be virtually invisible and bite humans. Straw itch mites are common in areas where grain is grown. They breed in stored grain and parasitize bugs that eat crops, making them beneficial pests. They occasionally hitch a ride on grains into human homes and can live in carpet and jump onto other surfaces. Their cousin, the dust mite, lives in dust and feeds off of dead skin cells. Though dust mites don't bite people directly, many people have allergic reactions to contact with dust mites. People with dust mite allergies may develop rashes that look like bug bites. Most species of mites are less than 0.1 millimeter in diameter, making them invisible to the naked eye.
Lice
Head lice are 1 to 2 millimeters and have translucent black or gray bodies. They feed on blood in hair shafts. Under a magnifying glass, the blood they have eaten is visible in their stomach. They are human parasites and live everywhere humans live. Their cousin the crab louse infests pubic hair and is slightly smaller, with a reddish-black body. They spread to humans through physical contact and through toilet seats, blankets and clothing. Both louse species are excellent jumpers.
Fleas
Several species of fleas live in the fur of mammals and feed on their blood. They range in size from 0.5 millimeters to 0.3 millimeters. Most fleas are black or brown, though the flea that most commonly attacks humans is translucent yellow. These insects are impressive jumpers, and can jump thousands of times their height. They can live up to three months without food, and they have a lifespan of up to two years, making them particularly difficult to get rid of once they have infested a person or animal.