These signs are often found in crevices of the body, such as on the webs of fingers and toes, around the genital area, in stomach folds of the skin, and under the breasts. Symptoms typically appear two to six weeks after infestation for individuals never before exposed to scabies. For those having been previously exposed, the symptoms can appear within several days after infestation. However, symptoms may appear after several months or years. Crusted scabies The elderly, disabled, and people with impaired immune systems, such as those with HIV/AIDS, cancer, or those on immunosuppressive medications, are susceptible to crusted scabies (also called Norwegian scabies). On those with weaker immune systems, the host becomes a more fertile breeding ground for the mites, which spread over the host's body, except the face. The mites in crusted scabies are not more virulent than in noncrusted scabies; however, they are much more numerous (up to two million). People with crusted scabies exhibit scaly rashes, slight itching, and thick crusts of skin that contain large numbers of scabies mites. For this reason, persons with crusted scabies are more contagious to others than those with typical scabies. Such areas make eradication of mites particularly difficult, as the crusts protect the mites from topical miticides/scabicides, necessitating prolonged treatment of these areas. Cause Scabies mite In the 18th century, Italian biologists Giovanni Cosimo Bonomo and 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 eight legs as adults, and are placed in the same phylogenetic class (Arachnida) as spiders and ticks. S. scabiei mites are under 0.5 mm in size, but are sometimes visible as pinpoints of white. Gravid 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 three to ten days. These young mites move about on the skin and molt into a "nymphal" stage, before maturing as adults, which live three to four weeks in the host's skin. Males roam on top of the skin, occasionally burrowing into the skin. In general, the total number of adult mites infesting a healthy hygienic person with noncrusted scabies is small, about 11 females in burrows, on average. The movement of