In the 18th century, Italian biologist Diacinto Cestoni (1637–1718) described the mite now called Sarcoptes scabiei, variety hominis, as the cause of scabies. Sarcoptes is a genus of skin parasites and part of the larger family of mites collectively known as "scab mites". These organisms have 8 legs as adults, and are placed in the same phylogenetic class (Arachnida) as spiders and ticks.
Sarcoptes scabiei are microscopic, but sometimes are visible as pinpoints of white. Pregnant females tunnel into the dead, outermost layer (stratum corneum) of a host's skin and deposit eggs in the shallow burrows. The eggs hatch into larvae in 3–10 days. These young mites move about on the skin and molt into a "nymphal" stage, before maturing as adults, which live 3–4 weeks in the host's skin. Males roam on top of the skin, occasionally burrowing into the skin. In general, there are usually few mites on a healthy hygienic person infested with non-crusted scabies; approximately 11 females in burrows can be found on such a person.
The movement of mites within and on the skin produces an intense itch, which has the characteristics of a delayed cell-mediated inflammatory response to allergens. IgE antibodies are present in the serum and the site of infection, which react to multiple protein allergens the body of the mite. Some of these cross-react to allergens from house-dust mites. Immediate antibody-mediated allergic reactions (wheals) have been elicited in infected persons, but not in healthy persons; immediate hypersensitivity of this type is thought to explain the observed far more rapid allergic skin response to reinfection seen in persons having been previously infected (especially having been infected within the previous year or two). Because the host develops the symptoms as a reaction to the mites' presence over time, there is usually a 4– to 6-week incubation period after the onset of infestation. As noted, those previously infected with scabies and cured may exhibit the symptoms of a new infection in a much shorter period, as little as 1–4 days.
Scabies is contagious, and can be spread by scratching an infected area, thereby picking up the mites under the fingernails, or through physical contact with a scabies-infected person for a prolonged period of time. Scabies is usually transmitted by direct skin-to-skin physical contact. It can also be spread through contact with other objects, such as clothing, bedding, furniture, or surfaces with which a person infected with scabies might have come in contact. Scabies mites can survive without a human host for 24 to 36 hours. As with lice, scabies can be transmitted through sexual intercourse even if a latex condom is used, because it is transmitted from skin-to-skin at sites other than sex organs.