Scabies

Scabies is a highly contagious infestation of the skin. It is caused by a mite, a tiny arachnid too small to see with the naked eye, called Sarcoptes scabiei var. hominis.

That mite is an obligate ectoparasite: it cannot live away from human skin. The female burrows a tunnel into the stratum corneum, the most superficial layer of the skin, the one made of cells that are already dead. In there she deposits her eggs and her fecal pellets, and there she ends up dying.

What comes next is not the mite's doing, it is the body's. Everything left buried in there is antigenic material, foreign molecules that the immune system recognizes, and against them it mounts a type IV hypersensitivity reaction, the kind called delayed. Out of that come the intense itching, which is worst at night, and the papules, those small raised bumps. The itching does not come from the mite directly: it comes from the body's reaction to what it leaves buried.