Critters that live on or in one. Furtive derives from the Latin fur, furtis meaning 'thief'.

Furtive Fauna: A Field Guide to the Creatures Who Live On You is a companion volume to Flattened Fauna and is also written by Roger M. Knutson. Penguin Books Ltd., 1992, ISBN 0 14 01.5378 0.

The fauna include:

Visitors
Ticks (Ixodidae) - "They can wait for years for a passing food source, which might include you".
Flies (Diptera)
Mosquitoes
Chiggers

Neighbors
The Common Bedbug (Cimex lectularius)
The Human Flea (Pulex irritans) - "They have the most elaborate sex lives of any animal and the relative ability to leap tall buildings in a single bound".
The Human Body Louse (Pediculus humanus corporis)
Dust Mites (Dermatophagoides farinae and Dermatophagoides pteronyssinus)

Residents
The Head Louse (Pediculus humanus capitis) - "You will acquire and maintain body lice by living in your clothes and washing them less than once a month. You get head lice by going to school".
Crab Louse or Pubic Louse (Phthirus pubis)
The Tropical Chigoe or Jigger (Tunga penetrans)
Itch Mites (Sarcoptes scabiei)
Face Mites or Follicle Mites (Demodex folliculorum and Demodex brevis) - "The good news is that they're microscopically small; the bad news is that they live exactly where their name implies".

Way Too Small to See
Dermatophyes (Tricophyton and Microsporum)
Hair Fungi (Tricosporon beigelii and others)
The Tooth Amoeba (Entameba gingivalis) - "What's the closest thing to stagnant water that's near us? We have all seen the TV commercials about morning mouth. That's stagnant water in there, folks, and we shouldn't be at all surprised that at least one amoeba lives in it".
Bacteria (Staphylococcus, Corynebacteria, and Micrococci)