Crusted scabies (previously called Norwegian scabies) is a highly contagious and severe form of infestation with Sarcoptes scabiei var hominis, most often seen in patients who are immunocompromised or institutionalized, or have a developmental disability. This form of scabies often occurs in patients with HIV infection, especially in those with low CD4 counts. It has also been reported as an IRIS (immune reconstitution inflammatory syndrome) phenomenon ("unmasking" type). The condition is also seen in leukemia-lymphoma patients and in transplant patients. Crusted scabies occurs worldwide in indigent populations affecting all age groups. It is particularly prevalent in healthy Aboriginal communities in Northern Australia.

Pruritus is a key symptom in both types of scabies; however, patients with crusted scabies may be completely asymptomatic. While the typical patient with scabies has fewer than 10-12 mites present, thousands to millions are present in the patient with crusted scabies. Spread of scabies occurs via skin-to-skin contact but also via fomites. The incubation period for crusted scabies may be as short as 4-5 days due to the high mite load.