This section is from the book "The Home Hand-Book of Domestic Hygiene and Rational Medicine. Volume 2.", by J. H. Kellogg, M.D.. Also available from Amazon: The Home Hand-Book of Domestic Hygiene and Rational Medicine, Volume 2.
Pediculosis, or lousiness, may result from the infesting of the body by any one of three varieties of the louse insect, or pediculus. They are known respectively, as the head louse, the body or clothes louse, and the crab louse or "crabs" See Figs. 340, 341 and 342 for representations of these three kinds of lice. Lice multiply very rapidly. A single female will produce in the course of a couple of months 5,000 new individuals. Very frequently an eruption may be seen, which is produced by the irritation of the insect and the scratching of the patient. Head lice deposit their eggs or "nits'' upon the hair, as shown in Fig. 343. Sometimes these are the only traces of the insect to be found. These "nits" cling very closely to the hairs to which they are attached until they are destroyed or hatched.
Fig. 340. The Head Louse.
Fig. 341. The Body or Clothes Louse.
Fig. 342. The Crab or Pubic Louse.
Fig. 343. Nits or Eggs of the Head Louse.
Lice do not usually exist except on persons who are filthy in their personal habits, although the most cleanly individual might become infested by contact with a person who harbored them in large numbers. Very frequently these parasites exist for a long time unsuspected. They can only be detected by careful scrutiny of the affected parts for the insects or their "nits." Body lice generally deposit their nits in the seams of the garments, and themselves usually cling to the clothing when it is removed.