Head Lice (Pediculus humanus capitis) are a common parasite found in human hair. About 1/16 to 1/8 inch long, they are small, wingless, grayish-white insects with flattened, elongated bodies, somewhat oval heads and sharp little claws perfectly evolved for what they do best—crawling along and clinging tenaciously to shafts of hair in between meals.

Infestations of head lice are generally accompanied by an intense itching of the scalp caused by the way they feed. Utilizing specially adapted mouthparts to cut through the skin, they literally "saw" through the scalp until they penetrate and rupture a blood vessel, whereupon they suck up blood until full.

Pediculus humanus capitis is an ectoparasite confined to the scalp and hair of humans. Infestations are prevalent worldwide and especially common among schoolchildren in both developed and developing countries (Gratz 1997).Infestations are found commonly among school children – especially in grades 1, 2, 3 [needs citation]. Unlike the human body louse, head lice have not been proven to be a vector of infectious disease agents (de Berker and Sinclair 2000). Lice are a common pest which are widely distributed throughout the world and infest both humans and animals. They are spread by contact and are a problem even under relatively sanitary conditions. Lice are insects about two to three millimeters in length. They lay eggs called nits, which look like white grains of sand and are firmly attached to the hairs by a cement like excretion.

The eggs generally hatch in about eight to ten days at body temperature. Human lice are divided into two genera: Head lice, Pediculus capitis or Pediculus humanus capitis, and Body lice or Clothing lice, Pediculus humanus humanus or Pediculus corporis; and Pubic lice, Phthirius pubis. Lice are spread by crowding and common usage of clothing and combs. Initially, infestations result at most in irritation, which can lead to infection of the irritated area, although there are at least three major diseases that are primarily transmitted by lice: epidemic typhus, trench fever and relapsing fever. Head lice are tolerant to a wide range of harsh environments [isopropyl alcohol (100%), salt (10%), chlorine (5 mg/L), and vinegar (100%)]. They are able to enter into a physiologically protective dormant condition or “stasis” for at least 20 min (average duration of most head lice treatments) during which they are impervious to most agents. Prevention using repellents or transmission inhibitors is thus an important option to reduce infestation opportunities. Head lice are very common and mainly affect children between 3 and 12 yr old. Little is known about the way nits, the eggs of the head louse, are attached to the hair.

Pediculosis capitis is caused by the head louse, Pediculus humanus capitis (De Geer 1776). The prevalence of head lice is usually about 10%, but in some regions one of every four children is infested (Gratz 1997, Downs 2000). Hundreds of millions of human infestations with head lice are thought to occur annually (Taplin & Meinking, 1987). Numbers have increased worldwide since the mid-1960s (Gratz, 1997) and an estimated 6-12 million people, mainly children, undergo some sort of treatment annually. Several compounds, such as mayonnaise, petroleum jelly and olive oil have erroneously been reported to kill head lice based on the less stringent criteria of efficacy, but in reality these caused only a transient period of stasis ( Burkhart & Burkhart, 2001, 2004, 2006a, 2006b ).

^United States Patent 5227163 Clilco, Ltd. Lice-repellant compositions 07/13/1993 International Journal of Dermatology 2007, 46, 422–426 A comparison of botanical and synthetic substances commonly used to prevent head lice (Pediculus humanus var. capitis) infestation Deon V. Canyon, PhD, MPH, and Rick Speare, BVSc, MBBS, PhD British Journal of Dermatology , 146 , 88 – 93 . Gratz , N.G . ( 1997 ) Human Lice: their Prevalence, Control and Resistance to Insecticides: a Review 1985 – 1997 .