Scabies is a common communicable skin infestation caused by a mite.

Scabies is a common communicable skin infestation caused by a mite. This mite, Sarcoptes scabiei, is sometimes known as the human itch or mange mite due to the intense itching caused when the pregnant female mite burrows into the top layer of skin and lays eggs.
Based on archeological evidence, including Egyptian drawings depicting people afflicted with signs of scabies, scabies is estimated to have been an infestation of humans for at least 2,500 years. There are many controversial accounts of the history of discovery of the infectious agent. Prior to the 17th century, the condition of scabies was known by many names and widely believed to be a humoral disease, possibly associated with a mite. Humoral diseases were part of an ancient theory that held that health came from balance between the bodily liquids. These liquids were termed humors. Disease arose when imbalance occurred between the humors. The term scabies is believed to be derived from the Latin term scabere, which means to scratch, or possibly from the term scabs, which are secondary to bacterial infection.
It is estimated that more than 300 million cases of scabies occur worldwide every year. Anyone who has had contact with the mite can catch scabies. Scabies infestations can affect people from all socioeconomic levels without regard to age, sex, race or standards of personal hygiene. Scabies spreads rapidly under crowded conditions where there is frequent skin-to-skin contact between people, such as hospitals, institutions, child-care facilities, and nursing homes. In young, healthy persons scabies is generally considered to be more of a nuisance than a disease. In elderly persons, or those who are immunocompromised, scabies is generally not diagnosed until cutaneous (skin) lesions and symptoms are apparent. Because of the long incubation period, many people can be exposed to scabies before the infested person is diagnosed.
The purpose of this manual is to provide schools, local health departments, 
Healthcare facilities, and other group settings a comprehensive guide to identify, treat, manage, and prevent scabies infestations. This handbook was designed to serve as a universal guide, providing technical information about scabies as well as a quick reference.
Life Cycle
The Sarcoptes scabiei mite (Figure 1) is an obligate parasite that lives in the skin. The adults are small (females 0.3 to 0.4 mm, males 0.25 to 0.35 mm in length) and rounded in shape, with tiny pointed spines on their dorsal surface that assist them in burrowing. Sarcoptes scabiei undergoes 4 stages in its life cycle; egg, larva, nymph and adult.
The adult mites can crawl rapidly on the surface of the skin, with females traveling up to 2.5 cm/min (~1 in/min). Upon finding a suitable site, the female
mite burrows into the skin, completely disappearing beneath the surface in about an hour. A saliva-like substance is secreted by the mite to aid in burrowing by dissolving the skin. Male and female mites mate within these burrows, after which the impregnated female emerges and excavates a permanent burrow
in which to lay her eggs; the male mite dies. The female mite will spend the rest of her life (commonly 30-60 days) in this permanent burrow and will continue to extend the length of the burrow, usually burrowing a total length of 1 cm (~1/2 inch) or more.
Shortly after digging her permanent burrow, the female mite begins laying eggs, producing 2 or 3 each day. The eggs hatch in 3-4 days, and within one day of hatching, the larvae begin actively crawling out of the burrow towards the surface of the skin. They then excavate shallow burrows in which they feed and molt to nymphs 
about 3 days later. The nymphs return to the skin surface or dig just beneath the surface, where they molt to adults in 3-4 days. The developmental time from egg to adult typically takes about 10 days for males and 14 days for females (Figure 2, top). Male mites live only 1-2 days and spend this time seeking out unmated females.
Although a female mite can lay as many as 180 eggs in her life time, fewer than 10% of her offspring live long enough to hatch and reach the adult stage. Most eggs are removed from the skin by bathing, scratching, or rubbing of the skin. Once away from the human body, mites do not survive more than 48-72 hours.
Sarcoptes scabiei infestation is specific to humans and is different from the mite infestations that affect dogs and other animals, which are more commonly known as mange. Mites from mange-infested animals can burrow into human skin but cannot reproduce, so they die within a few days.
Habits and Habitat
The most frequent S. scabiei burrow sites are in the folds of skin around the wrists and in the webbing between the fingers. Other common sites are the elbows, feet, and ankles; axillae; buttocks; genital regions; and for women, breasts (Figure 2, bottom). The location of burrows in infants and young children differs somewhat from that of adults, commonly involving the palms, sides, soles of the feet, and the head and neck region. Rashes may also occur on other parts of the body and are often caused by the burrowing of immature stages and unfertilized female mites. Unlike adults, children often develop rashes on the face, chest and back. Feeding activity and host immune system response to mite secretions and fecal matter are the sources of irritation that lead to scratching, scabbing, and subsequent secondary infections. 
The severity of scabies infestation is directly related to the number of mites residing on the skin and the length of time between initial infestation and subsequent diagnosis and treatment. Fewer than 10-15 mites may be present on an infested person who is otherwise healthy.
If diagnosis and treatment are delayed, the number of live mites multiplies resulting in heavier
or atypical infestations.