Scabies (from , "to scratch"), also known colloquially as the seven-year itch, is a contagious skin infection caused by the mite Sarcoptes scabiei. The mite is a tiny, and usually not directly visible, parasite which burrows under the host's skin, which in most people causes an intense itching sensation caused by an allergic response. The infection in animals other than humans is caused by a different but related mite species, and is called sarcoptic mange. Scabies is classified by the World Health Organization as a water-related disease. The disease may be transmitted from objects, but is most often transmitted by direct skin-to-skin contact, with a higher risk with prolonged contact. Initial infections require four to six weeks to become symptomatic. Reinfection, however, may manifest symptoms within as few as 24 hours. Because the symptoms are allergic, their delay in onset is often mirrored by a significant delay in relief after the parasites have been eradicated. Crusted scabies, formerly known as Norwegian scabies, is a more severe form of the infection often associated with immunosuppression.